An Etymological Dictionary of Astronomy and Astrophysics

1. The glowing envelope of gas and dust that surrounds a comet’s nucleus.
   

2. An elongated, → comet-shaped → image of a → point source of → light
   caused by → aberration in the → optical system. In → telescopes it is often because → off-axis rays of light striking different parts of the → objective do not focus in the same → image plane.
   → Coma Berenices; → Coma cluster; → hydrogen coma.

See also: L. coma “hair,” from Gk. kome “hair;” → hair.