Comatic aberration (or coma), in an optical system, refers to aberrations inherent in certain optical designs or due to imperfections in the lens or other components in which rays of light entering the objective away from the optical axis are not brought to focus in the same image plane. For example, stars appear distorted and appear to have a comet-like tail (coma).
Specifically, comatic aberration is defined as a variation in magnification across the entrance pupil. In refractive or diffractive optical systems, especially those imaging a wide spectral range, coma can be a function of wavelength, in which case it is a form of chromatic aberration.
It is a particular type of astigmatism that occurs when both the light source and the aperture are off the optical axis. The portions of the beam exiting the optical system are no longer elliptical, but take on a characteristic elongated comet shape (hence the name coma). An optical system corrected for coma aberration is said to be isoplanatic.