Technically, this word may be used to refer to any object which is orbiting another. However, it is most frequently used to refer to the primary vehicle component of the U.S. Space Transportation System, better known as the Space Shuttle. The Orbiter is the actual spacecraft, and as such carries a name (Columbia, Challenger, Atlantis, Discovery, Endeavour). It is mated with Solid Rocket Boosters and an External Tank to produce the Space Transportation System.