1. A celestial body, observed only in that part of its orbit that is relatively close to the sun, having a head consisting of a solid nucleus surrounded by a nebulous coma up to 2.4 million kilometers (1.5 million miles) in diameter and an elongated curved vapor tail arising from the coma when sufficiently close to the sun. comets are thought to consist chiefly of ammonia, methane, carbon dioxide, and water.

1. A celestial body consisting mainly of ice, dust and gas in a (usually very eccentric) orbit around the sun and having a "tail" of matter blown back from it by the solar wind as it approaches the sun.
2. A celestial phenomenon with the appearance given by the orbiting celestial body.

1. A member of the solar system which usually moves in an elongated orbit, approaching very near to the sun in its perihelion, and receding to a very great distance from it at its aphelion. A comet commonly consists of three parts: the nucleus, the envelope, or coma, and the tail; but one or more of these parts is frequently wanting. see illustration in appendix.

1. one of a class of celestial bodies which move about the sun in greatly elongated orbits, usually elliptical or parabolic.
2. in heraldry, same as blazing-star.
3. one of a group of humming-birds with long forked tails: as, the sappho comet, Cometes sappho; the Phaon comet, Cometes phaon.
4. A game of cards, somewhat like speculation, invented and popular in the reign of Louis xv. of france.
5.
6. in photography, a comet-shaped defect appearing on gelatin dry plates.

1. (astronomy) a relatively small extraterrestrial body consisting of a frozen mass that travels around the sun in a highly elliptical orbit