Keratoconus (KCKTCN) is a degenerative disorder of the eye in which structural changes within the cornea cause it to thin and change to a more conical shape than the more normal gradual curve. Keratoconus can cause substantial distortion of vision, with multiple images, streaking and sensitivity to light all often reported by the person. It is typically diagnosed in the person's adolescent years. If both eyes are significantly affected, the deterioration in vision can affect the person's ability to drive a car or read normal print.

In most cases, corrective lenses fitted by a specialist are effective enough to allow the person to continue to drive legally and likewise function normally. Further progression of the disease may require surgery, for which several options are available, including intrastromal corneal ring segmentscorneal collagen cross-linkingmini asymmetric radial keratotomy, corneal intrastromal implantation system (CISIS), topography-guided photorefractive keratectomy(PRK), topography-guided conductive keratoplastyphakic intraocular lenses and, in 25% of cases, corneal transplantation.

Estimates of the rates for keratoconus range from 1 in 500 to 1 in 2000 people,  but difficulties with differential diagnosis cause uncertainty. It seems to occur in populations throughout the world, although it is observed more frequently in certain ethnic groups, such as South Asians. Environmental and genetic factors are considered possible causes, but the exact cause is uncertain. It has been associated with detrimental enzyme activity within the cornea. The word is from Greekkéras horn, cornea; and Latincōnus cone.