20/20
Fraction commonly used to indicate normal vision. A bottom number larger than twenty indicates diminished vision. For example, a person with 20/400 vision would see a line of text that is twenty feet away with the same readability that a person with "normal" vision would see that line of text from 400 feet away.
A
A Scan
Ultrasonic procedure to measure specific areas within the eye.
AAO
See American Academy of Ophthalmology or the American Academy of Optometry.
ABES
See American Board of Eye Surgeons.
Ablation
Removal. In the case of the excimer laser, the frequency of energy causes the molecules of the cornea to loose the "glue" that holds them together. They simply fall away from each other and the remaining cornea.
Ablation Zone
The area of the cornea including the fully corrected optical ablation zone and the transition zone.
ABMD
See Anterior Basement Membrane Dystrophy.
ABO
See American Board of Ophthalmology.
Accommodation
The ability of the eye to change focus from distant objects to objects closer than optical infinity, approximately 20 feet (6 meters). Like when driving and you switch from looking down the road to reading the speedometer. Accommodation is achieved when the lens shape is changed by small muscles around the lens pushing and pulling. Loss of accommodation is called Presbyopia.
Accommodative Insufficiency
Loss of or less than expected amount of accommodation.
ACES
See American College of Eye Surgeons
Acuity
Clearness, as in visual acuity. The most common measure of visual acuity is the Snellen acuity chart. Normal acuity being 20/20 (6/6 metric version).
Adnexa
The accessory structures of the eye including the eyelids, lacrimal apparatus, etc.
Advanced Medical Optics
Ophthalmic equipment company.
Aesculap-Meditec
Ophthalmic equipment company. Click here to go to website.
Against-the-rule Astigmatism
"With the rule" astigmatism has an axis of about 180 degrees (when Rx is written in minus cylinder form). "Against the rule" astigmatism has the axis at about 90 degrees.

Both forms are common. Nevertheless, when looking at population norms, "with the rule" is more common in young adults while "against the rule" is more common in older adults.
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.
AK
See Astigmatic Keratotomy.
Alcon
Medical device manufacturer. Click here to go to website.
Algorithm
A precise mathematical rule or set of rules to find a solution to a problem. In refractive surgery, algorithm often refers to the mathematical calculations of the excimer laser used to create an ablation patter to reshape the cornea for the purpose of reducing the need for corrective lenses, such as in Lasik, PRK, LASEK, or Epi-Lasik.
ALK
See Automated Lamellar Keratectomy.
All-Laser Lasik
Lasik with the corneal flap being created by a femtosecond laser rather than with a mechanical microkeratome. See All-Laser Lasik details
Allegretto
Medical device manufacturer. Click here to go to website.
Allergan
Ophthalmic equipment and drug company. Click here to go to website.
Amblyopia
Often called "lazy eye" it is a unilateral (occasionally bilateral) condition in which, in the absence of any obvious structural anomalies or ocular disease, the best corrected vision is less than 20/20 (6/6). Amblyopia is often occurs in an eye that did not have adequate use during early childhood.

Most often amblyopia results from either a misalignment of a child's eyes, such as crossed eyes, or a difference in image quality between the two eyes (one eye focusing better than the other.) In both cases, one eye becomes stronger, suppressing the image of the other eye. If this condition persists, the weaker eye may becomes useless.

With early diagnosis and treatment, the sight in the "lazy eye" can be restored.

AMD
See Age-related Macular Degeneration.
American Academy of Ophthalmology
Ophthalmologist membership organization.
American Academy of Optometry
Optometrist membership organization.
American Board of Eye Surgeons
Ophthalmologist certification organization. Affiliated with the American College of Eye Surgeons. Click here to go to website.
American Board of Ophthalmology
A medical specialty board that administers a 1-1/2 year long education and examination process for ophthalmologists. After an ophthalmologist has passed the examination, (s)he is "Board Certified". Click here to go to website.
American College of Eye Surgeons
Ophthalmologist membership organization. Affiliated with the American Board of Eye Surgeons. Click here to go to website.
American College of Surgeons
Medical membership organization. Members are called Fellows. Physicians who are members often use the acronym FACS after their name. Click here to go to website.
American Optometric Association
Optometric membership organization. Click here to go to website.
American Society of Cataract and Refractive Surgery
Ophthalmologist membership organization. Click here to go to website.
Ametropia
Any imperfection in refractive state of the eye. Examples would be hyperopia (farsighted, longsighted vision), myopia (nearsighted, shortsighted vision), or astigmatism (irregular cornea).
AMO
See Advanced Medical Optics
Amsler Grid
A hand held chart featuring horizontal and vertical lines, usually white on black background, used to test for central visual field defects.
Aniseikonia
Aniseikonia is a binocular condition in which left and right images differ in size or shape. There are two types of aniseikonia – static and dynamic aniseikonia. The first type is the classical aniseikonia, denoting a perceived image size difference with a fixed gaze direction. Classic aniseikonia can occur naturally or be induced by refractive surgery.
The second type of aniseikonia is also called induced anisophoria and denotes a perceived image size difference due to unequal prism effects when looking through different parts of the two (anisometropic) spectacle lenses. Detailed Aniseikonia and Lasik Information
Anisometropia
The condition in which the two eyes have an unequal refractive power. One eye may be myopic (nearsighted, shortsighted) and the other hyperopic (farsighted, longsighted). Detailed Anisometropia and Lasik Information
Anophthalmia
Congenital absence of eyes.
Anopia
Absence of eyes.
Anterior
Front or forward portion.
Anterior Basement Membrane Dystrophy
Also called Epithelium Basement Membrane Dystrophy, is a disease that disrupts the ability of the epithelium to attach to the lower layers of the cornea. The epithelium will often grow unevenly or will detach from the cornea.
Anterior Chamber
The space in front of the iris and behind the cornea.
Anterior Ocular Segment
The part of the eye anterior to the crystalline lens, including the cornea, anterior chamber, iris and ciliary body
Antimetropia
A condition in which one eye is myopic and the fellow eye is hyperopic.
Antioxidants
Micro-nutrients that destroy or neutralize free radicals.
AOA
See American Optometric Association.
Aphakia
The absence of the eye's natural crystalline lens, usually after cataract removal.
Aphakic Spectacles
Thick, plus-powered eyeglasses that were once the standard correction for optical power following extraction of cataract. The glasses were cumbersome and greatly distorted peripheral vision. Today, an intraocular lens (IOL) is implanted in the eye after the cataract is removed.
Aqueous Fluid
See aqueous humor.
Aqueous Humor
Clear watery fluid that flows between and nourishes the lens and the cornea. It is secreted by the ciliary processes.
Argon Laser
Laser light produced from argon gas. The main wavelengths are 488.0 nm blue and 514.5-nm pea green light, but nine separate wavelengths in the blue-green visible light spectrum are produced.
ARMD
See Age Related Macular Degeneration.
ASCRS
See American Society for Cataract and Refractive Surgery.
Astigmatic Keratotomy
A surgical procedure in which microscopic incisions are placed in the peripheral cornea to create a more spherical shape. Similar to Radial Keratotomy.
Astigmatism
A condition in which the surface of the cornea is not spherical, but is irregularly shaped like the back of a spoon. An astigmatic cornea causes light images to focus on two separate points in the eye, creating a distorted image. Symptoms range from visual discomfort in mild cases, to severe blurring, ghosting, and distortion similar to a reflection in a fun-house mirror. The amount of astigmatism you have will appear in the second number of most glasses prescriptions. The angle of direction (cylinder meridian or axis) of the astigmatism will be shown in degrees. A prescription of -4.00-1.00x30°, would indicate that you have 1.00 diopter of astigmatism at 30 degrees axis.
Atopy
A type of inherited allergic response involving elevated immunoglobulin E. Sometimes called a reagin response, it means that you have hay fever, bronchial asthma, or skin problems like urticaria or eczema. It can also be acquired, sometimes following hepatitis or extended contact with solvents or alcohol.
Anisometropia
The absence of refractive error symmetry between both eyes. If one person's eye has a refractive error that is more than one diopter greater than the other, it is called anisometropia.
Anterior_Chamber
The anterior chamber is in the front of the eye behind the cornea and in front of the iris.
Automated Lamellar Keratoplasty
An incisional refractive surgery technique for low to moderate myopia (nearsighted, shortsighted vision). In the procedure, the refractive surgeon places an instrument called an automated microkeratome on the eye which removes, in a shaving motion, a thin layer of cornea only microns thick. An even thinner layer of cornea underneath this top cap is removed, and the top cap is replaced. The procedure does not require sutures.
Automated Perimeter
A computer-driven device used to plot defects in the visual field. Usually the patient's head is placed into this large hemisphere shell. 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.
Autonomous Technologies Corporation
Manufacturer of ophthalmic lasers.