Any of a number of defects that impair the image in an optical instrument. Aberration occurs because of minute variations in lenses and mirrors, and because different parts of the light spectrum are reflected or refracted by varying amounts.
In chromatic aberration the image is surrounded by colored fringes, because light of different colors is brought to different focal points by a lens. In spherical aberration the image is blurred because different parts of a spherical lens or mirror have different focal lengths. In astigmatism the image appears elliptical or cross-shaped because of an irregularity in the curvature of the lens. In coma the images appear progressively elongated toward the edge of the field of view.
The term was coined 1737 by French mathematician Alexis Clairaut (1713–1765).
See aberration, optical.