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• Adnexa

  The tissues and structures surrounding the eye: eyelids, orbit, extraocular muscles, and lacrimal system.

• Amblyopia

  Decreased visual acuity without any apparent disease of the eye.

• Ametropia

  Any optical error of the eye that can be corrected by glasses or contact lenses. See Refractive Error.

• Anterior Chamber

  The area between the inner-most layer of the cornea and the iris.

• Anterior Chamber Angle

  The junction of the cornea and the iris.

• Anterior Segment

  Front portions of the eye including the cornea, the anterior chamber, the iris, and the crystalline lens.

• Aphakia

  Absence of the crystalline lens of the eye, usually as a result of cataract surgery.

• Aqueous Humor

  Clear watery fluid, which fills the anterior chamber of the eye.

• Astigmatic Keratotomy (AK)

  Surgical procedure where incisions are made in the cornea to correct astigmatism.

• Astigmatism

  Distortions in the cornea, or sometimes the lens, that focus light rays at different lengths, making it difficult to focus well at any distance.

• Automated Lamellar Keratoplasty (ALK)

  A surgical procedure for correcting high myopia by removing and folding back a layer of the anterior cornea, removing a precise amount of corneal tissue with an automated microkeratome, and folding back into position the first layer without stitches.

• Autorefraction

  An automated objective refraction that measures each eye individually without patient response or interaction.

• Axis

  An axis is a line dividing a regular figure symmetrically. As used in optometry, two axes define the direction of the longest and shortest radii of an oval (astigmatic) lens system of the eye. Common usage refers to the longer axis of a lens, the direction of least power, as the cylinder axis.