Generally, a band of asteroids which encircles a star, often where a planet would normally have formed. It may form the main "world" in a stellar system, as in the Barnard Belt, or it may simply be a region between planets where no planet coalesced, as in the Sol Belt.
Asteroids belts have a number of possible origins. Generally, a planet failed to coalesce during the formation of its star system, usually because of gravitational forces from a nearby gas giant, or because of the disruptive influence of multiple stars (Belts are for this reason far more common in systems with multiple suns than in one star systems).
Much more rarely, a planet has been torn apart by tidal forces, leaving a path of scattered debris. There are also a few instances when "planet-buster" weapons were used, such as Muno Gamma, destroyed by Geminga Orthodoxy during the Second Empires War.
Asteroid belts usually include several thousand asteroids, along with billions of smaller fragments, some as tiny as grains of dust, which are often the result of collisions between asteroids. Because of the ease of extracting resources, asteroid belts are highly sought after by developers.