Astigmatism
Common vision problem and type of refractive error. Caused by either irregularity in the curvature of the cornea or the lens of the eye. People with astigmatism generally have difficulty seeing fine details at all distances. Treated with corrective lenses, laser vision correction or toric IOLs.

Bifocals
Eye glasses that combine two lenses made for focusing at different distances. Typically the upper lens provides clear distance vision and the lower lens clear close vision for reading and seeing fine details.

Cataract
Clouding of the eye's lens that blocks passage of light to the retina, resulting in impaired vision. Often a result of normal aging, cataracts form when protein clumps cloud areas of the eye's lens. As the cataract progresses, vision worsens and often requires surgical replacement of the damaged lens with an artificial one.

Farsighted, farsightedness (or hyperopia)
Common vision problem and type of refractive error. Caused by too little curvature of the cornea or too little distance between the front of the eye and the retina at the back. Both structural defects cause light entering the eye to focus incorrectly on the retina, resulting in blurred close-up vision. Treated with corrective lenses, laser vision correction or multifocal or accommodative IOLs.

Intraocular lens (IOL)
Artificial lens made of plastic, silicone or acrylic. Designed to be implanted in the eye in place of or in front of the natural lens to improve focus and correct vision problems, such as cataracts and presbyopia.

LASIK (laser in-situ keratomileusis) surgery
Type of laser surgery in which the cornea is reshaped to improve vision. Either a microkeratome or a femtosecond laser is used to surgically create a thin, hinged flap of corneal tissue. The flap is folded back, and an excimer laser is directed to the corneal surface exposed beneath the flap to reshape the cornea for corrected vision. Then the flap is brought back into place.

LASIK Monovision
LASIK surgery to correct vision so that one eye focuses clearly on far objects and the other eye focuses clearly on near objects. The brain combines the two images to create clear vision at all distances.

Lens
The transparent disc behind the pupil that brings light into focus on the retina. As the eye ages, the lens often becomes cloudy and is called a cataract.

Nearsighted, nearsightedness (or myopia)
Common vision problem and type of refractive error. Caused by either too much curvature of the cornea or too much distance between the front of the eye and the retina at the back. Both structural defects cause light entering the eye to focus incorrectly on the retina, resulting in blurred distance vision. Treated with corrective lenses, laser vision correction or multifocal or accommodative IOLs.

Presbyopia
Also called age-related focus dysfunction. Common vision problem that develops naturally over time. Characterized by loss of the eye's ability to focus at close distances or on fine details. Treated with reading glasses, contact lenses, presbyopia laser vision correction (also called LASIK monovision) or presbyopia lens replacement surgery.

Refractive error
Irregularities in the cornea, lens or shape of the eye that prevent correct focus at any distance.

Refractive surgery
Any surgical procedure that attempts to decrease the patient's refractive error (irregularities in the eye that prevent correct focus). LASIK surgery is a common type of refractive surgery as is lens replacement.