co·ma

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.
• noun Botany A usually terminal tuft or cluster, especially a tuft of hairs on a seed, as on a willow or milkweed seed.
• noun Physics A diffuse, comet-shaped image of a point source of light or radiation caused by aberration in an optical system.
• noun A state of deep, often prolonged unconsciousness, usually the result of injury, disease, or poison, in which an individual is incapable of sensing or responding to external stimuli and internal needs.