What sort of disorder is a corneal abrasion?

K abrasion. This is an injury of the surface layer of the cornea called the epithelium as a result of a cut or scratch. This may be secondary to a fingernail, paper cut, tree branch, etc.
Abrasion of cornea. The cornea, the front window of the eye, is covered by a thin epithelium. When this is disturbed, usually by an object scraping it off such as a hairbrush bristle, or any object entering the front of the eye, an abrasion occurs. These are painful, cause the eye to get quite red, and usually disturb the vision. See your ophthalmologist as soon as possible for diagnosis and relief.