Ectasia - A progressive corneal thinning and bulging.

Emmetropes - People who have no refractive error.

Emmetropia - The ophthalmic term for a perfect refractive state - no nearsightedness, farsightedness or astigmatism.

Endothelium - The innermost layer of the cornea. The endothelium is one cell layer thick (5-10 microns or .005-.01 millimeters) and provides hydration balance to maintain the cornea's transparency. The endothelium serves three main purposes it regulates the stroma's water content, provides a barrier to ingress of several constituents of the aqueous humor, and actively transports glucose.

Enhancement - A secondary refractive procedure performed after the initial one in an attempt to achieve better visual acuity.

Enzymatic cleaner - A cleaner that removes protein deposits and other debris from contact lenses. It's recommended for use either daily, weekly, or monthly. Some enzymatic cleaners are a small tablet dropped into a solution along with the lens; others come in liquid form.
 

Epithelial ingrowth - A complication of LASIK wherein epithelial cells grow underneath the corneal flap.

Epithelium - The outermost layer of cells of the cornea. Six cells thick (20 microns), the epithelium is the eye's first defense against infection.

Excimer laser - A "cold" laser used in refractive surgery to remove corneal tissue 

Farsightedness - See "Hyperopia"

Ferrara rings - Intrastromal segments implanted into patient's cornea tissue. Invented by Paolo Ferrara from Brazil, the diameter is 5 mm and the cross section is triangular and works like a prism reflecting away light that hits the rings.
 

Fibroblast - A fibroblast is a cell that makes the structural fibers and ground substance of connective tissue. Fibroblasts make collagens, glycosaminoglycans, reticular and elastic fibers, and glycoproteins found in the extracellular matrix.
 

Fleischer's ring - Ferritin (iron) deposition around the base of the cone, seen in moderately advanced KC cases.

Floaters - A dark or gray spot that passes across the field of vision causing you to move your eye. Floaters are very common and are caused by the undissolved vitreous humor that floats in the liquid vitreous. Sometimes, a "shower of floaters" is a sign of a serious condition, particularly if you also see flashes of light.