Astigmatism
From The Glossary of Digital Photography
A type of lens aberration in which horizontal and vertical lines are not in focus at the same point. The human eye often contains an astigmatism. As many as one-third of the population suffers from astigmatism, and it tends to increase with age. Just as the proper glasses can adjust for astigmatism in the eye, lens systems can be designed to eliminate it in photographic applications. See also aberration
