The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber, providing most of an eye's optical power. Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainsville, Florida: Triad Publishing Company, 1990. Together with the lens, the cornea refracts light, and as a result helps the eye to focus, accounting for approximately 80% of its production to 20% of the lens focusing power. Goldstein, E. Bruce. "Sensation & Perception". 7th Edition. Canada: Thompson Wadsworth, 2007. The cornea contributes more to the total refraction than the lens does, but, whereas the curvature of the lens can be adjusted to "tune" the focus depending upon the object's distance, the curvature of the cornea is fixed and restored.

The cornea has unmyelinated nerve endings sensitive to touch, temperature and chemicals; a touch of the cornea causes an involuntary reflex to close the eyelid. Because transparency is of prime importance the cornea does not have blood vessels; it receives nutrients via diffusion from the tear fluid at the outside and the aqueous humour at the inside and also from neurotrophins supplied by nerve fibres that innervate it. In humans, the cornea has a diameter of about 11.5 mm and a thickness of 0.5 mm - 0.6 mm in the center and 0.6 mm - 0.8 mm at the periphery. Transparency, avascularity, and immunologic privilege makes the cornea a very special tissue. The cornea is the only part of a human body that has no blood supply; it gets oxygen directly through the air.

In humans, the refractive power of the cornea is approximately 43 dioptres, roughly two-thirds of the eye's total refractive power.

Medical terms related to the cornea often start with the prefix "kerat-".

It borders with the sclera by the corneal limbus.