Pronunciation: (u-stig'mu-tiz"um),[key] —n. 1. Also called a•stig•mi•aPronunciation: (u-stig'mē-u).[key]Ophthalm.a refractive error of the eye in which parallel rays of light from an external source do not converge on a single focal point on the retina.
2. Optics.an aberration of a lens or other optical system in which the image of a point is spread out along the axis of the system.