The definition of a comet is a celestial body that moves around the sun, and as it goes near the sun it forms a vapor trail of dust and gas that streams after it.
An example of comet is Haley's.
The McNaught comet at sunset.
comet definition by Webster's New World
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
Origin: Middle English comete ; from Old English cometa and amp; Old French comete, both ; from Classical Latin cometa ; from Classical Greek komētēs, literally , long-haired Americanism ; from komē, literally , hair of the head
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.
Origin: Middle English comete, from Old English comēta, from Late Latin, from Latin comētēs, from Greek komētēs, long-haired (star), comet, from komē, hair.
comˈet·arˌy , co·metˈic adjective
Word History: Comets have been feared throughout much of human history, and even in our own time their goings and comings receive great attention. Perhaps a comet might seem less awesome if we realized that our name for it is based on a figurative resemblance between it and humans. This figurative name is recorded first in the works of Aristotle, in which he uses komē, the Greek word for “hair of the head,” to mean “luminous tail of a comet.” Aristotle then uses the derived word komētēs, “wearing long hair,” as a noun meaning “comet.” The Greek word was adopted into Latin as comētēs, which was refashioned in Late Latin and given the form comēta, furnishing Old English with comēta, the earliest English ancestor of our word comet.
An object that enters the inner solar system, typically in a very elongated orbit around the sun. Material is boiled off from the comet by the heat of the sun, so that a characteristic tail is formed. The path of a comet can be in the form of an ellipse or a hyperbola. If it follows a hyperbolic path, it enters the solar system once and then leaves forever. If its path is an ellipse, it stays in orbit around the sun.
Comets were once believed to be omens, and their appearances in the sky were greatly feared or welcomed.
The most famous comet, Comet Halley (or Halley's comet), passes close to the Earth roughly every seventy-six years, most recently in 1986.
A celestial object that orbits the Sun along an elongated path. A comet that is not near the Sun consists only of a nucleus—a solid core of frozen water, frozen gases, and dust. When a comet comes close to the Sun, its nucleus heats up and releases a gaseous coma that surrounds the nucleus. A comet forms a tail when solar heat or wind forces dust or gas off its coma, with the tail always streaming away from the Sun. ♦ Short-period comets have orbital periods of less than 200 years and come from the region known as the Kuiper belt. Long-period comets have periods greater than 200 years and come from the Oort cloud. See more at Kuiper belt, Oort cloud. See Note at solar system.