Petzval field curvature
An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. 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. The term comes from the Greek α- (a-) meaning "without" and στίγμα (stigma), "a mark, spot, puncture".
Other articles related to "astigmatic":
... pencil does not meet the refracting or reflecting surface at right angles therefore it is astigmatic (Gr ... The interval O'O", termed the astigmatic difference, increases, in general, with the angle W made by the principal ray OP with the axis of the system, i.e ... Two astigmatic image surfaces correspond to one object plane and these are in contact at the axis point on the one lie the focal lines of the first ...