comet

n. celestial body with a luminous tail

A comet is a small body in the solar system that orbits the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus, which itself is a minor body composed of rock, dust, and ice. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into Sun-grazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
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Noun
1. (astronomy) a relatively small extraterrestrial body consisting of a frozen mass that travels around the sun in a highly elliptical orbit
(hypernym) extraterrestrial object, estraterrestrial body
(part-meronym) coma
(classification) astronomy, uranology

(n.)
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.
  

Comet [from Greek komet long-haired, alluding to the cometary tail] A stage in the formation of globes from the primordial world-stuff, following the state known as the comic curds and preceding the formation of suns and planets. "What does Science know of Comets, their genesis, growth, and ultimate behaviour? Nothing . . . And what is there so impossible that a laya centre -- a lump of cosmic protoplasm, homogeneous and latent, when suddenly animated or fired up -- should rush from its bed in Space and whirl throughout the abysmal depths in order to strengthen its homogeneous organism by an accumulation and addition of differentiated elements? And why should not such a comet settle in life, live, and become an inhabited globe!" (SD 1:204). They are called wanderers, and some of them become suns, others planets. Some become attracted to solar systems and pursue closed orbits because they are reimbodying planets; others have not yet assumed periodic form; more are either broken up or absorbed by the influence of neighboring suns or globes. The matter of which they are composed, though on the same plane albeit in its higher portions, as our senses (otherwise they would not be visible to us), is not of the same kind as our terrestrial matter, but they are on their way towards it during their ages of condensation.