Anterior chamber – The space in the eye that is behind the cornea and in front of the iris.

Astigmatism – A common form of visual impairment in which part of an image is blurred, due to an irregularity in the curvature of the front surface of the eye, the cornea.

Cataract – A clouding of the lens of the eye. The normally clear aspirin-sized lens of the eye starts to become cloudy which will eventually impair normal vision.

Cornea – The clear front window of the eye that transmits and focuses light into the eye.

Crystalline lens – A transparent, colorless body located in the front third of the eyeball, behind the iris, that helps bring rays of light to a focus on the retina.

Custom LASIK – The newest wave in LASIK involves measuring the eye from front to back, using what's called "wavefront" technology, to customizing the treatment to your individual visual irregularities, beyond nearsightedness, farsightedness and astigmatism.

Diabetic retinopathy – A common complication of diabetes affecting the blood vessels in the retina. If untreated, it may lead to blindness.

Diopter – Unit of measure for optical strength or refractive power of glasses or contact lenses.

Endothelial cells – Refers to the cells that make up the thin, innermost layer of the cornea. Endothelial cells are essential in keeping the cornea clear.

Farsightedness (hyperopia) – The ability to see distant objects more clearly than close objects. It is the result of an eyeball that is too short or whose outside surface (the cornea) is too flat. The exact cause is not known, although farsightedness may be inherited.

Fovea – The central point on the retina that produces the sharpest vision.

Functional vision –The ability to see clearly while performing daily activities in varying levels of light.

Glaucoma – A common eye condition in which the fluid pressure inside the eyes rises because of slowed fluid drainage from the eye. If untreated, it may damage the optic nerve and other parts of the eye, causing the loss of vision or even blindness.

Halos – Hazy ring around bright lights seen by some patients with refractive error or optical defects (e.g., cataracts or corneal swelling). This symptom may occur after surgery.

Intraocular lens – A small synthetic lens, made of a hard plastic (polymethyl methacrylate or PMMA), silicone, acrylic, or hydrogel material implanted into an eye after cataract surgery to replace crystalline lens, which has been removed.

Intraocular Pressure (IOP) – Pressure caused by the fluid inside the eye; it helps to maintain the shape of the eye.

Iris – Circular, colored portion of the eye. Its opening forms the pupil. The iris helps regulate the amount of light that enters the eye.

LASIK – Laser-assisted in situ keratomileusis. A laser assisted surgical procedure to correct for the refractive error of the cornea. A tiny flap is cut in the top of the cornea, underlying corneal tissue is removed with an excimer laser, and the flap is put back in place; used to correct nearsightedness, farsightedness, and astigmatism.

Microkeratome – The instrument a surgeon uses to create the corneal flap in the uppermost layer of the cornea during the LASIK procedure.

Monofocal IOL – Provides good distance and sometimes intermediate vision but doesn't correct near vision. Usually requires corrective lenses or eyeglasses after surgery for reading and near vision tasks.

Multifocal IOL – Designed to provide distance and near vision simultaneously but does not restore good intermediate vision. Increased glare and halos reported at night.

Nearsightedness (myopia) – The ability to see close objects more clearly than distant objects. In this type of refractive error, light is focused in front of the retina. Correction of nearsightedness brings the light to focus on the retina.

Ophthalmologist – A medical doctor (M.D.) specializing in refractive, medical, and surgical treatment of eye diseases and disorders.

Posterior chamber – The fluid-filled space between the back of the iris and resting against the capsular bag.

Presbyopia – The natural crystalline lens of the eye loses its ability to “accommodate,” meaning its ability to switch from seeing objects at a distance (one focal point) to seeing near objects (second focal point).

Pupil – The opening at the center of the iris of the eye that varies in diameter depending upon the brightness of the light coming into the eye.

Refractive surgery – Refers to many different procedures used for correcting the refractive error of an eye.

Retina – The thin membranous lining of the rear two-thirds of the eye that converts images from the eye’s optical system into electrical impulses sent along the optic nerve for transmission to the brain.

Retinal detachment – A separation of the retina from its connection at the back of the eye. The separation usually results from a tear in the retina.

Spherical aberrations – The deviation of light rays through lenses that cause images to be blurred.

Uncorrected Visual Acuity (UCVA) – The measurement of visual acuity (such as 20/20) without wearing glasses or contact lenses.

Visual Acuity – The clarity or clearness of the vision, a measure of how well a person sees. The ability to distinguish details and shapes of objects; also called central vision.

Visual aberrations – A defect of focus, such as blurring in an image.