Glossary of Eye-related Terms

Ablation
Surgical removal of tissue, typically using a cool beam laser.

Ablation zone
The area of tissue that is removed during laser surgery.

Accommodation
Ability of the eye to change its focus between distant objects and near objects.

Acuity
Sharpness, acuteness, or keenness of vision.

Acute
Occurring suddenly.

Adnexa
Accessory structures of the eye, including the eyelids, lacrimal apparatus, etc.

AK
Astigmatic Keratotomy, modified form of Radial Keratotomy (RK).

Amblyopia
Dullness or obscurity of sight for no apparent organic reason, therefore not correctable with glasses or surgery. Sometimes called a lazy eye, wherein one eye becomes dependent on the other eye to focus, usually developed in early childhood. Often associated with strabismus.

Amsler grid
Hand held chart featuring horizontal and vertical lines, usually white on black background, used to test for central visual field defects.

Angle
Drainage area of the eye formed between the cornea and the iris, named for its angular shape, which is why you see the word "angle" in the different glaucoma names.

Anisometropia
Condition of the eyes in which they have unequal refractive power.

Anterior chamber
Space between the cornea and the crystalline lens, which contains aqueous humor.

Anterior ocular segment
Part of the eye anterior to the crystalline lens, including the cornea, anterior chamber, iris and ciliary body.

Antioxidants
Micro nutrients that destroy or neutralize free radicals, molecules that have been implicated as one causative factor in the stimulation of abnormal cellular reproduction (cancer) and cellular destruction (aging).

Anti-reflective coating
Coating on the front or back of glasses lenses, which minimizes the glare for patients who are extremely bothered by glare.

Aphakia
Absence of the lens of the eye.

Aqueous humor
Transparent fluid occupying the anterior chamber and maintains eye pressure.

Argon laser
device used to treat glaucoma (usually open angle) and diabetic retinopathy using a thermal beam.

ARMD
age related macular degeneration: Destruction and loss of the photoreceptors in the macula region of the retina resulting in decreased central vision and, in advanced cases, blindness.

Asthenopia
Eyestrain.

Astigmatic Keratotomy (AK)
Treats astigmatism by flattening the cornea with arc-shaped incisions in its periphery, similar to RK.

Astigmatism
Structural defects of the eye in which the light rays from a viewed object do not meet in a single focal point, resulting in blurred images being sent to the brain. An astigmatic cornea is not perfectly rounded like a basketball but has an irregular shape more like the side of a football. Astigmatism is most often combined with myopia or hyperopia (read more).

Automated Lamellar Keratoplasty (ALK)
Procedure that predates LASIK eye surgery and is not generally used any more.

Automated perimeter
Computer-driven device used to plot defects in the visual field (peripheral vision or side vision). Usually, this is a large hemisphere shell into which the patient's head is placed. Various points of lights, sometimes of different sizes, intensities and colors are projected onto the screen. The patient then indicates whether the light is seen and the response is recorded. The computer then plots the effective visual thresholds within the targeted visual field.

Axis
Optical - a straight line through the centers of both surfaces of a lens. Visual - a straight line from the object of vision to the fovea of the eye.

BCVA
Best corrected visual acuity.

Best corrected visual acuity (BCVA)
Best possible vision a person can achieve with corrective lenses, measured in terms of Snellen lines on an eye chart.

Beta-carotene
Member of the carotinoid family of vitamins, a precursor to vitamin A, thought to be beneficial to the eyes, helpful in treating diseases such as glaucoma.

Bifocals
Lenses containing two focal lengths, usually arranged with the focus for distance above and near focus below.

Binocular vision
Simultaneous use of the two eyes. Normal binocular vision yields a stereoscopic image and parallax-induced depth perception.

Blepharitis
Inflammation of the eyelids, a common problem which tends to be reoccurring in nature (read more).

Blind spot
The area of the optic disk where the optic nerve fibers exit the eye and where there are no light-sensitive cells. This small area can be measured and in glaucoma, as the nerve fibers die, the blind spot tends to enlarge and elongate. This is one of the diagnostic hallmarks of glaucoma.

Bowman's membrane
Extremely thin second layer of the cornea, situated between the epithelium and stroma, thought to be responsible for epithelium adhesion.

Capsular haze
A thin film of scar tissue that occasionally forms on the posterior capsule behind the intraocular lens implant following cataract surgery and removed with a Nd:Yag laser.

Caruncle
Small, red portion of the corner of the eye that contains modified sebaceous and sweat glands.

Cataract
Gradual clouding of the crystalline lens resulting in reduced vision or eventual blindness, correctable by cataract surgery (read more).

Cataract surgery
Removal of a cataract, replacing it with an intraocular lens implant (read more).

Central islands
Central islands are a small mound of central tissue, which can interfere with vision and occur when the laser beam does not uniformly remove tissue in the center of the treatment.

Choroid membrane
Dark, vascular, thin skin-like tissue, situated between the sclera and the retina, forming the middle coat of the eye. The choroid membrane nourishes the outer portions of the retina and absorbs excess light.

Chronic
Of long duration, going on for some time.

Closed angle glaucoma
Glaucoma conditions occurring suddenly (acute).

Ciliary body
Part of the eye that connects the choroid membrane to the iris. Produces aqueous humor that fills the front part of the eye and maintains eye pressure.

Ciliary muscle
Muscle attached to the crystalline lens responsible for focus (the same as ciliary body, but used in a different context).

Clear Lens Extraction (CLE)
Procedure in which the eye's natural clear crystalline lens is removed and replaced with an intraocular lens implant, using the same technique as cataract surgery (read more).

Color blindness
Inaccurate term for a lack of perceptual sensitivity to certain colors. Absolute color blindness is almost unknown.

Color vision
Ability to perceive differences in color, including hue, saturation and brightness.

Comprehensive eye exam
Evaluation of the complete visual system (read more).

Conductive Keratoplasty (CK)
Procedure in which a radio frequency probe, rather than a laser, is used to reshape the cornea. It is approved for low to moderate hyperopia in patients over age 40, however it does not appear to have the precision of LASIK.

Cones
One of the two types of light-sensitive cells, concentrated in the center of the retina (also see rods). There are about 6.5 million cones in each eye - 150,000 cones in every square millimeter - responsible for detailed visual acuity and the ability to see in color.

Conjunctiva
Mucous membrane lining the inner surface of the eyelids and covering the front part of the sclera (white part of eye), responsible for keeping the eye moist.

Conjunctivitis
Inflammation or irritation of the conjunctiva. Symptoms can be present in just one eye, or it can affect both eyes and include redness of the eyes or the edges of the eyelids, swelling of the eyelids or itching.

Contact lens
Small, thin removable plastic lens worn directly on the front of the eyeballs, usually used instead of ordinary eyeglasses for correction or protection of vision.

Convergence
Turning of the eyes inwards so that they are both "aimed" towards a near object being viewed. Normally works in harmony with divergence which is used for more distant objects.

Cornea
Transparent tissue that forms the front part of the eyeball, covering the iris and pupil. The cornea is the first part of the eye that bends (or refracts) the light and provides most of the focusing power.

Corneal curvature
Shape of the front of the eye.

Corneal mapping, topography
A tool used to see the refractive problems that might be present in the cornea. Corneal topography is used not only for screening all patients before refractive surgery like LASIK but also for fitting contacts.

Corneal relaxing incisions (CRIs)
True corneal incisions, such as RK and AK.

Corneal transplant (penetrating keratoplasty)
Surgical operation of grafting a replacement cornea onto an eye.

Crystalline lens
Double convex, transparent part of the eye, located behind the iris and in front of the vitreous body. Serves in conjunction with the cornea to refract incoming rays of light onto the retina.

Cylinder
Refers to the degree of astigmatism (uneven roundness) present in the cornea.

Depth perception
Ability of the vision system to perceive the relative positions of objects in the visual field.

Detached retina
A retinal detachment occurs when the retina is pulled away from its normal position in the back of the eye.

Diabetes mellitus
Chronic metabolic disorder characterized by a lack of insulin secretion and/or increased cellular resistance to insulin, resulting in elevated blood levels of simple sugars (glucose) and including complications involving damage to the eyes, kidneys, nervous system and vascular system

Diabetes type I (IDDM)
Insulin dependent, resulting from destruction of the insulin producing pancreatic islet cells

Diabetes type II (NIDDM)
Non-insulin dependent, resulting from tissue resistance to insulin

Diabetic retinopathy
Deterioration of retinal blood vessels in diabetic patients, possibly leading to vision loss (read more).

Dilated, dilation
Enlargement of the pupil (space in the middle of the iris).

Diopter
Unit of measure of the refractive power of an optical lens (equal to the power of a lens with a focal distance of one meter). A negative diopter value (such as -3D) signifies an eye with myopia and positive diopter value (such as +3D) signifies an eye with hyperopia.

Diplopia
Condition in which a single object is perceived as two; also called double vision.

Divergence
Turning of the eyes outwards so that they are both "aimed" towards the object being viewed. Normally works in harmony with convergence.

Double vision
Same as diplopia.

Dry eye
A common condition that occurs when the eyes do not produce enough tears to keep the eye moist and comfortable (read more).

Emmetropia
Twenty-twenty vision.

Endothelium
Cellular tissue that covers the inner surface of the eye within the closed cavity, typically referring to the cornea.

Enhancement
An additional LASIK procedure, used in the refinement of Personal Best Vision.

Epithelium
Cellular tissue that covers the outer surface of the eye. Consists of one or several layers of cells with only little intercellular material.

Esophoria
Position of the eyes in an over-converged position compensated by the external eye muscles so that the eyes do not appear turned inward.

Esotropia
Position of the eyes in an over-converged position so that non-fixating eye is turned inward. One eye looks straight; one looks inward.

Excimer laser
Laser used in LASIK surgery that operates in the ultraviolet wavelength, producing a cool beam.

Exophoria
Position of the eyes in an over-diverged position compensated by the external eye muscles so that the eyes do not appear turned outward.

Exotropia
Position of the eyes in an over-diverged position so that non-fixating eye is turned outward. One eye looks straight ahead and one turns outward.

Extracapsular cataract surgery
Surgery in which the cataract is removed in one piece through a larger incision, which usually requires several stitches.

Extraocular muscles
Six muscles that control eye movement. Five originate from the back of the orbit; the other one originates from the lower rim of the orbit. Four move the eye up, down, left and right, the other two control the twisting motion of the eye when the head tilts. All six muscles work in unison; when they do not function properly, the condition is called strabismus.

Eye chart
Technically called a Snellen chart, a printed visual acuity chart consisting of Snellen optotypes, which are specifically formed letters of the alphabet arranged in rows of decreasing letter size.

Eyelid
Either of two movable, protective, folds of flesh that cover and uncover the front of the eyeball.

Farsighted
Common term for hyperopia.

FDA
Abbreviation for the Food and Drug Administration. It is the United States governmental agency responsible for the evaluation and approval of medical devices.

Femtosecond laser
Used in the IntraLASIK procedure to make a safer and more precise flap than the older mechanical microkeratome technology, it uses a longer wavelength, smaller spot, and shorter duration per pulse than the excimer laser used to reshape the cornea.

Field of vision
Entire area which can be seen without shifting the gaze.

Flap
Part of the cornea consisting of epithelium, Bowman's membrane and some stroma, cut with a remaining hinge and lifted up as part of the LASIK procedure.

Flashes & floaters
Light spots or streaks and dark moving specks due to the vitreous traction on the retinal (light flashes) and solid vitreous material or blood (floaters) (read more).

Fluorescein angiography
Diagnostic test by which the veins deep inside the eye are examined. Dye is injected into a vein in the arm and circulated by the blood to the back of the eye, allowing for visual examination.

Fovea
Small depression in the retina, the point where vision is most acute.

Fundus
Furthest point at the back of the eye, consisting of the retina, choroid membrane, sclera, optic disc and blood vessels, seen by means of the ophthalmoscope.

Ghost image
Faint second image of the object you are viewing.

Giant papillary conjunctivitis
Type of conjunctivitis wherein bumps or ridges form on the inside of eyelids, which make wearing contact lenses uncomfortable; in fact, this condition is often caused by over wearing of certain contact lenses

Glare
Scatter from bright light that decreases vision.

Glaucoma
Painless disease of the eye characterized by increased pressure within; left untreated it leads to a gradual impairment of sight often resulting in blindness (read more).

Gonioscopy
Viewing procedure utilizing a mirror/lens device placed directly upon the cornea that is used to view the drainage area called "the angle" through which aqueous fluid exits the eyeball.

Halos
Rings around lights due to optical imperfections in, or in front of, the eye.

Haptics
The arms of an intraocular lens, which holds it in place once inserted inside the eye.

Haze
Corneal clouding that causes the sensation of looking through smoke or fog.

Heterophoria
Constant tendency of one eye to deviate in one or another direction due to imperfect balance of ocular muscles.

Holmium laser
A laser which operates in the infrared wavelength, producing a hot beam. It is used in Laser Thermokeratoplasty surgery and more commonly in surgical procedures involving the disintegration of stones and fibrous tissue ablation.

Hyperopia
Also called farsightedness, hyperopia is the inability to see near objects as clearly as distant objects, and the need for accommodation to see distant objects clearly (read more).

Hypoxia
Deficiency of oxygen supply to a tissue.

Image
Light reflected into the eye, off objects in front of the eye. This light contains all the information about the objects (such as color, shadow. motion and detail) that are translated to the brain and allow you to "see" (know about the objects).

Inflammation
Body's reaction to trauma, infection, or a foreign substance, often associated with pain, heat, redness, swelling, and/or loss of function.

Informed Consent Form
Document disclosing the risks, benefits, and alternatives to a procedure.

In Situ
Term meaning "in place".

Intracapsular cataract surgery
Cataract surgery in which both the lens and capsule are completely removed, a rarely used procedure.

Intraocular lens implant (IOL)
Permanent, artificial lens surgically inserted inside the eye to replace the crystalline lens following cataract surgery or clear lens extraction.

Intraocular pressure (IOP)
Fluid pressure within the eye created by the continual production and drainage of aqueous fluid in the anterior chamber.

Iridotomy
Treatment for closed-angle glaucoma, one of the many types of glaucoma, usually done with a laser.

Iris
Colored part of the eye. Elastic, pigmented, muscular tissue in front of the crystalline lens that regulates the amount of light that enters the eye by adjusting the size of the pupil in the center.

Ischemia
Restriction or blockage of blood flow through a blood vessel. Ischemia is a causative agent of certain heart attacks and strokes and is involved in various types of visual field losses.

Intacs
Surgically implanted plastic half rings that change the shape of the cornea (read more).

Keratectomy
Surgical removal of corneal tissue.

Keratitis
Inflammation of the cornea

Keratotomy
Surgical incision (cut) of the cornea. .

keratoconus
Rare, serious, degenerative corneal disease, in which the cornea thins and assumes the shape of a cone.

Keratomileusis
Carving of the cornea to reshape it.

Keratoplasty
Surgical reshaping of the cornea.

Lacrimal apparatus
Part of the eye that produces tears.

LASEK
Laser Epithelial Keratomileusis, a refractive surgery in which the epithelium is cut with a fine blade, called a trephine, and involves displacing the corneal epithelium as a sheet and then replacing it to (theoretically) act as a natural bandage (read more).

Laser
Device that generates an intense and highly concentrated beam of light. Acronym for: Light Amplification by Simulated Emission of Radiation. (Also see: holmium laser, argon laser, Nd:YAG laser, femtosecond laser, and excimer laser)

Laser Thermokeratoplasty (LTK)
Holmium 'hot' beam laser, instead of the 'cool' beam excimer laser, is used to treat farsighted patients and is very limited in its application; the effects are not long lasting (read more).

LASIK
Laser Assisted In-Situ Keratomilieusis, a refractive surgery in which Excimer laser ablation is performed under a flap on the cornea to correct refractive errors(read more).

Lazy eye
Amblyopia, an eye condition noted by reduced vision not correctable by glasses or contact lenses and is not due to any eye disease.

Legally blind
20/200 vision and over is the qualification of legal blindness in the United States.

Lens
Same as the crystalline lens. Double convex, clear part of the eye, behind the iris and in front of the vitreous humor. Serves to refract the various rays of light so as to form an image on the retina.

Lenticular
Special non-cataract lenses for patients who have cataracts.

Lid speculum
A surgical tool that holds the eyelids open and which allows the surgeon to gain access to the eye with minimal pressure on the globe.

Limbal relaxing incisions (LRI)
Small incisions placed on the far peripheral aspect of the cornea resulting in a cornea that is more round, for correcting astigmatism (read more).

Limbus
Thin area that connects the cornea and the sclera.

Low vision
Condition occurring when ordinary eyeglasses or contact lenses are unable to bring a patient's sight up to normal sharpness.

LTK (Laser Thermal Keratoplasty)
Holmium 'hot' beam laser, instead of the 'cool' beam excimer laser, is used to treat farsighted patients and is very limited in its application; the effects are not long lasting (read more).

Lutein
Member of the carotinoid family of vitamins, similar to beta-carotene, thought to be beneficial to the eyes, helpful in treating diseases such as glaucoma.

Macula
Yellow spot on the retina, where the photoreceptors are most dense and responsible for the central vision. Has the greatest concentration of cones, responsible for visual acuity and the ability to see in color.

Macular edema
Collection of fluid in and under the macular portion of the retina.

Macular degeneration
Disease of the macula, which results in the loss of central vision (read more).

Meridian
Orientation of a particular curve, often used in relation to the cornea.

Microkeratome
Mechanical surgical device that is affixed to the eye by use of a vacuum ring. When secured, a very sharp blade cuts a layer of the cornea at a predetermined depth.

Miosis
Pupillary constriction.

Monovision
Purposeful adjustment of one eye for near vision and the other eye for distance vision (read more).

Mydriasis
Pupillary dilation.

Myopia
Also called nearsightedness or shortsightedness, the inability to see distant objects as clearly as near objects (read more).

Near point of accommodation
Closest point in front of the eyes that an object may be clearly focused.

Near point of convergence
Maximum extent the two eyes can be turned inwards.

Nearsighted
Common term for myopia.

Neodymium YAG Laser
Laser used to treat Posterior Capsular Opacification (PCO) as well as open angle glaucoma Selective Laser Trabeculoplasty

Neovascularization
Often associated with diabetes, involves the formation of new blood vessels, often fragile and inappropriate for the location.

Nerve fibers/axons
Extensions of photoreceptors that form the nerve bundle that is called the optic nerve.

Neuro-ophthalmology
Subspecialty that treats the nervous and vascular systems that involve the eye.

Normal vision
Occurs when light is focused directly on the retina rather than in front or behind it (read more).

Ocular herpes
A recurrent viral infection caused by the herpes simplex virus. Ocular herpes represents the most common infectious cause of corneal blindness in the United States.

Ocular hypertension
Elevated fluid pressure. The normal pressure is about 10 to 21mmHg, with the majority of people falling between 13 and 19. Over 21 is considered suspicious. Over 24 cautiously concerned - warranting immediate investigation. Over 30 is considered urgent and a potential emergency situation.

OD
Abbreviation standing for "oculus dextrum" meaning: right eye.

ONH
Optic nerve, optic nerve head. A bundle of nerve fibers about the diameter of pencil that passes through the back of the eyeball, and connects to the nerve fiber layer of the retina. It can be observed directly with an instrument called an ophthalmoscope.

Open angle glaucoma
Glaucoma conditions of long duration (chronic).

Ophthalmologist
An ophthalmologist is either a medical doctor (MD) or an osteopathic physician (D.O.) who is qualified and especially trained to diagnose and treat all eye and visual system problems, both medically and surgically, as well as diagnose general diseases of the body.

Ophthalmoscope
Instrument used to examine the interior of the eye: it consists of a perforated mirror arranged to reflect light from a small bulb into the eye.

Ophthalmoscopy
Examination of the internal structures of the eye using an illumination and magnification system.

Optic disc
The head of the optic nerve that is formed by the meeting of all retinal nerve fibers.

Optic nerve
Bundle of nerve fibers that connect the retina with the brain. The optic nerve carries signals of light to the area of the brain called the visual cortex, which assembles the signals into images called vision.

Optician
Expert who designs, verifies and dispenses lenses, frames and other fabricated optical devices upon the prescription of an ophthalmologist or an optometrist.

Optometrist
Eye care professional, graduate of optometry school, provides non-surgical visual care. Specifically educated and trained to examine the eyes, and determine visual acuity as well as other vision problems and ocular abnormalities. An optometrist prescribes glasses and contact lenses to improve visual acuity.

Orbit
Bony socket containing the eyeball, fat, extraocular muscles, nerves and blood vessels.

Orthokeratology (OK)
Non-surgical procedure using contact lenses to alter the shape of the cornea to effect a change in the refractive error.

Orthoptics
Exercises designed to help the eye muscles work together to improve visual perception.

OS
Abbreviation standing for "oculus sinistrum" meaning: left eye

Overcorrection
Occurrence in refractive surgery where the achieved amount of correction is more than desired; in LASIK, typically due to a patent's over-response to the laser ablation.

Pachymeter
Instrument that measures the distance between the top of the corneal epithelium and the bottom of the corneal endothelium used as diagnostic testing device measuring for corneal thickness.

Pachymetry
Exam for measuring corneal thickness.

Papilledema
Non-inflammatory swelling/elevation of the optic nerve often due to increased intracranial pressure or space-occupying tumor.

PD
Used on prescriptions to indicate the distance between the pupils of both eyes.

Pellucid marginal degeneration
A bilateral, noninflammatory, peripheral corneal thinning disorder, which is characterized by a peripheral band of thinning of the inferior cornea.

Peripheral vision
Ability to perceive the presence, motion, or color of objects outside the direct line of vision.

Personal Best Vision
Best possible vision for each individual as corrected.

Phacoemulsification cataract surgery
Cataract removal procedure which involves making a tiny incision, about 1/8" long. A pen-like instrument, inserted through the opening, is used to emulsify and aspirate the clouded lens material, using gentle sound waves. Then an intraocular lens is inserted into place.

Phacofracture cataract surgery
Cataract surgery in which the lens is removed through a small incision by "fracturing" it into several small segments, rarely used today.

Phakic Intraocular Lenses (IOLs)
Placed inside the eye without removing the natural lens, and performs much like an internal contact lens.

Phoropter
A common device found in most eye doctor's offices, with multiple lenses, used to measure refractive errors. A phoropter calculates the prescription required for corrective lenses.

Photocoagulation
Focusing of powerful light rays onto tiny spots on the back of the eye, producing heat, which seals retinal tears and cauterizes small blood vessels.

Photophobia
Sensitivity to light.

Photoreceptors
Microscopic light-sensitive cells that are located in the retina called rods and cones. There are approximately 7 million cones and 125 million rods

Photo Refractive Keratectomy (PRK)
Surgery in which a small area on the corneal epithelium (surface cells) is gently polished away. The laser then reshapes the corneal surface in exactly the same way as for LASIK surgery (read more).

Pingecula
Irritation caused degeneration of the conjunctiva resulting in a thickening and yellowing of the normally thin transparent tissue.

Pink eye
Type of conjunctivitis, commonly seen in children.

Posterior capsule
The thin membrane in the eye that holds the crystalline lens in place.

Posterior chamber
The back section of the eye's interior.

Posterior optical segment
Part of the eye posterior (behind) to the crystalline lens, including the vitreous, choroid retina and optic nerve.

Posterior Vitreous Detachment (PVD)
Separation of the vitreous body from its attachment from the retinal surface due to shrinkage from degenerative or inflammatory conditions or trauma. An age-related condition.

Prelex
Surgical procedure that attempts to correct presbyopia (read more).

Presbyopia
Inability to maintain a clear image (focus) as objects are moved closer. Presbyopia is due to reduced elasticity of the lens with increasing age (read more).

Prescription
Amount of vision correction necessary, written in a form that can be utilized during the manufacture of corrective lenses or to configure a laser machine (read more).

PRK
Acronym for Photo-Refractive Keratectomy, which is a procedure involving the removal of the surface layer of the cornea (epithelium) by gentle scraping and use of a computer-controlled excimer laser to reshape the stroma (read more).

Progressive lenses
Bifocal or trifocal lenses which appear to be single vision with no distinct lines between the various focal lengths.

Punctal occlusion
Treatment for dry eye in which plugs are inserted into the punctum in order to retain lubricating tears naturally produced by the eye.

Punctum
The hole in the upper and lower eyelids through which tears exit the eye. In patients with dry eyes, temporary or permanent plugs may be inserted to help keep tears in the eye. Tears flow through the punctum to the nose, which is why people often experience a runny nose when crying.

Pupil
Black circular opening in the center of iris through which light passes into the crystalline lens. It changes size in response to how much light is being received by the eye, larger in dim lighting conditions and smaller in brighter lighting conditions.

Pupillary response
Constriction and dilation of the pupil due to stimulation by light or accommodation.

Radial Keratotomy (RK)
Outdated procedure once used to correct mild to moderate myopia, whereby making a series of spoke-like incisions around its periphery flattens the cornea.

Refract
To bend aside, as in "the crystalline lensrefracts the light as it passes through", or to measure the degree the eyes or lenses bend light, as in "the doctor refracts a patient's eyes".

Refraction
Test to determine the refractive power of the eye; also, the bending of light as it passes from one medium into another.

Refractive errors
The degree of visual distortion or limitation caused by inadequate bending of light rays, includes hyperopia, myopia, and astigmatism.

Refractive power
Ability of an object, such as the eye, to bend light as light passes through it.

Refractive surgery
Type of surgery (such as LASIK) that affects the refraction of vision.

Retina
Layer of fine sensory tissue that lines the inside wall of the eye, composed of light sensitive cells known as rods and cones. Acts like the film in a camera to capture images, transforms the images into electrical signals, and sends the signals to the brain by way of the optic nerve.

Retinal Detachment
Condition wherein retina breaks away from the choroid membrane, causing it to lose nourishment and resulting in loss of vision unless successfully surgically repaired.

RK
Abbreviation for "radial keratotomy", an outdated procedure once used to correct mild to moderate myopia, whereby making a series of spoke-like incisions around its periphery flattens the cornea.

Rods
One of the two types of light-sensitive cells, located primarily in the side areas of the retina (also see cones). There are about 125 million rods, which are responsible for visual sensitivity to movement, shapes, light and dark (black and white) and the ability to see in dim light.

Routine eye exam
To test the overall condition of the eye and prescribe corrective measures such as glasses, contact lenses or LASIK.

Schirmer test
Test for dry eyes, which uses a thin strip of filter paper placed at the edge of the eye.

Sclera
White part of the eye. Tough covering that (with the cornea) forms the external, protective coat of the eye.

Scotoma
Area of partial or complete loss of vision surrounded by an area of normal vision, as what can occur in advanced ARMD or glaucoma.

Single vision
Lenses with only one focal length.

Slit-Lamp
Ophthalmic instrument producing a slender beam of light used to illuminate and examine the external and internal parts of the eye.

Sloan eye chart
A common chart used to test visual acuity with black letters of various sizes against a white background.

Snellen eye chart
Most common chart used to test visual acuity with black letters of various sizes against a white background.

Snellen lines
Snellen optotypes arranged in horizontal rows called "lines".

Snellen optotypes
Specifically formed letters of the alphabet arranged in rows of decreasing letter size on the Snellen chart.

Sphere
Focusing power of the corrective lens.

Stereoscopic vision
Ability to see in three-dimension.

Stereopsis
Ability to perceive three-dimensional depth.

Strabismus
Condition occurs when the muscles of the eye do not aligned properly and binocular vision is not present. Patients with a history of strabismus may develop double vision after refractive eye surgery.

Stroma
Middle, thickest layer of tissue in the cornea.

Suppression
Inability to perceive all of part of objects in the field of vision of one eye.

Suspensory ligament of lens
Series of fibers that connect the ciliary body of the eye with the lens, holding it in place; ; also known as zonules.

Sympathetic ophthalmia
Inflammation of one eye following inflammation in the other eye.

Tonometry
Procedure for the measurement of intraocular pressure. A test for glaucoma.

Topography
A tool used to see the refractive problems that might be present in the cornea. Corneal topography is used not only for screening all patients before refractive surgery like LASIK but also for fitting contacts.

Toric
Lens (eyeglasses, intraocular lens, or contact lens) that is the warped (astigmatic) opposite to that of the eye, thereby canceling out the error.

Trabecular meshwork
Drainage channels located inside the eye.

Trabeculoplasty
A procedure for the treatment of glaucoma, using a laser (Argon or Nd:YAG). Trabeculoplasty remodels the trabecular meshwork in order to increase drainage of aqueous and lower the intraocular pressure.

Trifocals
Lenses containing three focal lengths, usually arranged with the focus for distance above, intermediate distance in the middle, and near vision below.

Twenty-twenty, 20/20 vision
To have 20/20 vision means that when you stand 20 feet away from the Snellen eye chart you can see what the majority of people can see at that same distance (read more).

UCVA
Uncorrected visual acuity.

Uveal tract
Pigmented, middle layers of the eye, which include the choroid, ciliary body and iris.

Uveitis
Inflammation of any portion of the uveal tract.

Ultrasound waves
Sound waves above 20,000 vibrations per second, above the range audible to the human ear, used in medical diagnosis and surgery.

Ultrasonography
Recordings of the echoes of ultrasound waves sent into the eye and reflected from the structures inside the eye or orbit. Ultrasonography is used to make measurements and to detect and localize tumors and retinal detachments.

Ultraviolet radiation
Radiant energy with a wavelength just below that of the visible light. UV-c is the shortest wavelength at 200-280 nm and is absorbed by the atmosphere before reaching the surface. UV-b, at 280-315 nm is the burning rays of the sun and damages most living tissue. UV-a, at 315-400 nm is the tanning rays of the sun and is somewhat damaging to certain tissues. UV radiation has been described as a contributing factor to some eye disease processes, which result in ARMD and cataracts and causes exposure keratitis.

Uncorrected visual acuity (UCVA)
Best possible vision a person can achieve without corrective lenses measured in terms of Snellen lines on an eye chart.

Undercorrection
Occurrence in refractive surgery where the achieved amount of correction is less than desired; in LASIK, typically due to a patient under-responding to the laser treatment.

Vascular
Having to do with transporting blood.

Vision
The ability of the brain to see and interpret what is in front of the eyes.

Vision therapy
Orthoptics, vision training, eye exercises. Treatment process for the improvement of visual perception and/or coordination of the two eyes, for more efficient and comfortable binocular vision.

Visual acuity
Clearness of vision; the ability to distinguish details and shapes, which depends upon the sharpness of the retinal image.

Visual cortex
That part of the brain responsible for vision.

Visual field
Area or extent of space visible to an eye in a given position of gaze. There is a central visual field - the area directly in front of us, and a peripheral visual field - our "side vision". The fields of each eye partly overlap. We do not perceive the blind spots from each eye because the area that is missing in one eye is present in the other.

VISX CustomVue Procedure
WaveScan-driven laser vision correction with the potential to produce better vision than is possible with glasses or contact lenses, and enable surgeons to measure and correct unique imperfections in each individual's vision (read more).

VISX STAR S4 Excimer laser System
Highly advanced laser technology platform (see picture), the VISX STAR S4 combines Variable Spot Scanning (VSS) and ActiveTrak 3-D Active Eye Tracking along with the WavePrint(read more).

Vitreous humor, fluid, or body
Jelly-like, colorless, transparent substance occupying the greater part of the cavity of the eye, and all the space between the crystalline lens and the retina.

Vitrectomy
Surgical removal of vitreous humor that is diseased or has lost its transparency.

Wavefront
Wavefront technology produces a detailed map of the eye. The information is transferred to the laser via computer software (read more).

YAG laser surgery
Properly called Nd:Yag laser capsulotomy, a procedure using a Nd:YAG (neodymium-yttrium-aluminum-garnet) laser, used primarily to treat secondary cataracts (capsular haze) that occur subsequent to the primary cataract procedure, or to relieve increased pressure within the eye from acute angle-closure glaucoma via a peripheral iridotomy. It can also be used to treat open angle glaucoma in a procedure called selective lasertrabeculoplasty (read more).