The chamber is the part of a
firearm that holds the
ammunition to be fired. Not to be confused with the
magazine (more well known by the misnomer
clip), which holds the rest of the ammunition.
Revolvers have several chambers--anywhere from three to twelve, with five or six being most common--which rotate to line up with the barrel to fire, while automatic pistols have only one--after firing, the first round is ejected, and the next one is stripped from the magazine and loaded into the chamber to be fired. Most rifles and shotguns also have only one chamber.