The cornea is the transparent front part of the eyeball which covers the iris, pupil, and anterior chamber.

Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power.

Medical terms related to the cornea often start with the prefix "kerat-" from the Greek word κέρας, horn.

Transparency, avascularity, the presence of immature resident immune cells, and immunologic privilege makes the cornea a very special tissue.

In fish, and aquatic vertebrates in general, the cornea plays no role in focusing light, since it has virtually the same refractive index as water.

This arises at death, but a dead eye can be placed in a warm chamber with a reservoir of sugar and glycogen that generally keeps the cornea transparent for at least 24 hours.

When damage to the cornea occurs, such as in a viral infection, the collagen used to repair the process is not regularly arranged, leading to an opaque patch (leukoma).

In 2009, researchers at the University of Pittsburgh Medical center demonstrated that stem cell collected from human corneas can restore transparency without provoking a rejection response in mice with corneal damage.

Recently emerging tissue engineering technologies are expected to be capable of making one cadaver-donor's corneal cells be expanded and be usable in more than one patient's eye.

Its continuous irrigation with a tear fluid also results in poor retention of the therapeutic agents on the ocular surface.

Poor permeability of the cornea and quick wash out of therapeutic agents from ocular surface result in very low bioavailability of the drugs administered via topical route (typically less than 5%).

[30] If the corneal stroma develops visually significant opacity, irregularity, or edema, a cornea of a deceased donor can be transplanted.

[31] Many countries have years-long waitlists for corneal transplant surgery due to the shortage of donated corneas.

[32][33] Only a handful of countries consistently have a large enough supply of donated corneas to meet local demand without a waitlist, including the United States, Italy, and Sri Lanka.