A comet is a Small Solar System Body that orbits the Sun. When close enough to the Sun, a comet exhibits a visible coma (fuzzy "atmosphere"), and sometimes a tail, both because of the effects of solar radiation upon the comet's nucleus. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, ranging from a few kilometers to tens of kilometers across. The word "comet" came to the English language through the Latin cometes from the Greek word komē, meaning "hair of the head"; Aristotle first used the derivation komētēs to depict comets as "stars with hair." The astronomical symbol for comets () accordingly consists of a disc with a hairlike tail.

Comets have a variety of different orbital periods, ranging from a few years, to hundreds of thousands of years, while some are believed to pass only once through the inner Solar System before being thrown out into interstellar space. Short-period comets are thought to originate in the Kuiper Belt, or associated scattered disc,

Comets are distinguished from asteroids by the presence of a coma or tail, though very old comets that have lost all their volatile materials may come to resemble asteroids (see extinct comets). Asteroids are also believed to have a different origin from comets, having formed in the inner Solar System rather than the outer Solar System, but recent findings have somewhat blurred the distinction between asteroids and comets (see centaurs and asteroid terminology). Comets leave a trail of debris behind them. If the comet's path crosses Earth's path, then at that point may be meteor showers as Earth passes through the trail of debris. The Perseid meteor shower occurs every year between August 9 and August 13 when Earth passes through the orbit of the Swift–Tuttle comet. Halley's comet is the source of the Orionid shower in October.

Nucleus

They are often popularly described as "dirty snowballs", though recent observations have revealed dry dusty or rocky surfaces, suggesting that the ices are hidden beneath the crust (see Debate over comet composition]]). Comets also contain a variety of organic compounds; in addition to the gases already mentioned, these may include methanol, hydrogen cyanide, formaldehyde, ethanol and ethane, and perhaps more complex molecules such as long-chain hydrocarbons and [[amino acids.