The only requirements are that the pistol in question be small, light, and slim enough to easily conceal on the person, to avoid printing (the firearm showing through the clothing). The handgun need not be in a pocket holster. In some cases a handkerchief is sufficient to secure a sufficiently small handgun in a pocket.
Pocket pistols are generally semi-automatic; these firearms can be made thinner than a revolver of comparable ballistic strength, due to the lack of a cylinder. However, revolvers have been used as pocket pistols, especially small framed revolvers in calibers less than or equal to the .38 Special.
Derringers in weaker calibers were commonly used as pocket pistols.
The prototypical pocket pistol was the "Baby Browning", a small semi-automatic handgun made in .25 ACP. Many gun companies have made handguns in .25 ACP, which are easily concealed in the pocket.
Pocket pistols were made in calibers of .22 LR, .32 ACP, .380 ACP, various .32 caliber revolver rounds and .38 Special. Modern pocket pistols can now also come in 9x19mm Parabellum, and small .38 Special revolvers are still popular for concealed carry, as well as for use as backup guns. Small .357 Magnum revolvers also can be used as a pocket pistol.
Pocket pistols are not normally known for their potency. A pocket pistol is intended to emphasize first and foremost concealability over the ballistic strength of the round or possible volume of fire.