Front part of the part of the eye that separates the front eye chamber from the environment and is continuous for light rays.
The cornea is relatively thin (0.5-0.6mm) and transparent. Any change in transparency or curvature inevitably leads to a deterioration in vision. Once clouding is present, it usually does not regress and the only thing that helps is to replace it with a healthy, clear cornea, i.e. a corneal transplant, also known as keratoplasty.
Everyone can donate their cornea, regardless of their age or whether they see well or poorly or have to wear glasses. Whether a donated cornea can be used for transplantation depends on its nature.
https://de.wikipedia.org/wiki/Hornhaut (Wikipedia CC-by-sa-3.0)