Originally, any firearm designed to be fired one-handed. In contemporary American usage, the term "pistol" is limited to handguns in which the barrel and chamber form a single unit, i.e., self-loaders and single shots. It is considered erroneous to apply the term "pistol" to a revolver.
However, this distinction is of fairly recent origin. Colt referred to its revolvers as "pistols" at least into the 1880's, and the British military officially designated their service revolvers as "pistols" as late as the WWII era. The origin of this word is particularly obscure. Some trace it to the Italian town of Pistoia, an early gunmaking center. Another explanation derives from a Bohemian handgun called a "pist'ala" from a Czech word for "pipe." Several other more fanciful etymologies exist.