Keratoconus is a degenerative condition of the eye that involves thinning of the cornea (the transparent, front surface of the eye).  As a result of the thinning the normal shape of the cornea becomes distorted and a cone-shaped bulge develops; resulting in a progressive blurring of the vision.  People who suffer from keratoconus may wear glasses or, more often, hard/rigid contact lenses are used to treat their blurry vision.