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.

Cornea
Clear, curved surface at the front of the eye through which light enters the eye. Along with the sclera (the white part of the eye), provides external protection for the eye. Often called the window of the eye. During many types of vision correction surgery, such as LASIK, the cornea is reshaped to reduce or eliminate the main types of refractive error - nearsightedness, farsightedness and astigmatism.

Epithelial defects
Defects in the epithelium (protective layer of cells on the cornea) can allow invasion by bacteria, fungi, viruses or Acanthamoeba. Can be initiated by mechanical trauma or nutritional deficiencies. Symptoms are progressive redness, foreign body sensation, ache, sensitivity to light and tearing.

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.

Glare
Appearance of additional luster around lights that can interfere with vision, especially night vision. Sometimes occurs as a side effect of LASIK surgery and often decreases with time. May also be caused by cataracts or other eye conditions.

Halos
Appearance of rings around lights at night. Sometimes occur as a side effect of LASIK surgery and often decreases with time. May also be caused by cataracts or other eye conditions.

Higher order aberrations
More complex irregularities of the eye that can cause vision problems such as difficulty seeing at night, glare, halos, starbursts or double vision. Some small degree of higher order aberrations are normal and do not usually cause noticeable vision problems.

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.

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.

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

Starbursts
Appearance of rays or fine filaments of light coming off a light source, such as a headlight of a car. Can become longer and denser at night. Severity of starbursts varies-they may be experienced differently in terms of size, shape, length of rays, the density of rays and transparency (whether the light source is visible through the starburst). Sometimes occur as a side effect of LASIK surgery and often decreases with time. May also be caused by cataracts or other eye conditions