Astigmatism

In optics, astigmatism (from Greek: α- a- "without" + στίγματος stigmatos, gen. of στίγμα stigma "a mark, spot, puncture") is when an optical system has different foci for rays that propagate in two perpendicular planes. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances.