In American English, a pocket pistol is any small, pocket-sized semi-automatic pistol (or less commonly referencing either derringers, or revolvers), and is suitable for concealed carry in a pocket or a similar small space.
Pocket pistols are sometimes categorized as smaller than sub-compact pistols, but the distinction is not clear-cut as some small sub-compact pistols may be categorized as pocket pistols, and some large pocket pistols may be classified as sub-compact pistols.
A pistol is a type of handgun, characterized by a barrel with an integral chamber. The word "pistol" derives from the Middle French pistolet (c. 1550), meaning a small gun or knife, and first appeared in the English language c. 1570 when early handguns were produced in Europe. In colloquial usage, the word "pistol" is often used as a generic term to describe any type of handgun, inclusive of revolvers (which have a single barrel and a separate cylinder housing multiple chambers) and the pocket-sized derringers (which are often multi-barrelled).
The most common type of pistol used in the contemporary era is the semi-automatic pistol. The older single-shot and lever-action pistols are now rarely seen and used primarily for nostalgic hunting and historical reenactment. Fully-automatic machine pistols are uncommon in civilian usage because of their generally poor recoil-controllability (due to the lack of a buttstock) and strict laws and regulations governing their manufacture and sale (where they are regarded as submachine gun equivalents).