1. Astronomy The nebulous luminescent cloud surrounding the nucleus of a comet and composed of material evaporated from the nucleus when the comet is near the perihelion of its orbit. The nucleus and coma together form the head of a comet.

2. Botany A usually terminal tuft or cluster, especially a tuft of hairs on a seed, as on a willow or milkweed seed.

3. Physics A diffuse, comet-shaped image of a point source of light or radiation caused by aberration in an optical system.