Real Dictionary

comet in American English
a small, frozen mass of dust and gas revolving around the sun in a parabolic or elliptical orbit: as it nears the sun it vaporizes, forming a coma and, usually, a long tail of ions that points away from the sun
noun: a celestial body moving about the sun, usually in a highly eccentric orbit, consisting of a central mass surrounded by an envelope of dust and gas that may form a tail that streams away from the sun

comet in British English
noun: a celestial body that travels around the sun, usually in a highly elliptical orbit: thought to consist of a solid frozen nucleus part of which vaporizes on approaching the sun to form a gaseous luminous coma and a long luminous tail