A comet is a small Solar System body that orbits the Sun and, when close enough to the Sun, exhibits a visible coma (atmosphere) or a tail — both primarily from the effects of solar radiation upon the comet's nucleus. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, measuring a few kilometres or tens of kilometres across.

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 through the inner Solar System only once before being thrown out into interstellar space. Short-period comets are thought to originate in the Kuiper Belt, or associated scattered disc,Comets - Relics from the birth of the Solar System, Björn Davidsson, Uppsala University, 2008 which lie beyond the orbit of Neptune. Long-period comets are believed to originate at a very much greater distance from the Sun, in a cloud (the Oort cloud) consisting of debris left over from the condensation of the solar nebula. Comets are thrown from these outer reaches of the Solar System inwards towards the Sun by gravitational perturbations from the outer planets (in the case of Kuiper Belt objects) or nearby stars (in the case of Oort Cloud objects), or as a result of collisions.

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 the Earth passes through the trail of debris. The Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes through the orbit of the comet Swift-Tuttle. Halley's comet is the source of the Orionid shower in October.

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. Asteroids are also believed to have a different origin from comets, having formed in the inner Solar System rather than the outer Solar System. Recent findings have, however, somewhat blurred the distinction between asteroids and comets; see also Asteroid: Terminology.

There are a reported 3 535 comets as of June 2008, of which several hundred are short-period. This number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population: the reservoir of comet-like bodies in the outer solar system may number one trillion. The number of naked-eye comets averages to roughly one per year, though many of these are faint and unspectacular. When a historically bright or notable naked-eye comet is witnessed by many people, it is often considered a Great comet.