A Picatinny Rail is the common name for a U.S. military standard for a mounting rail system for rifles and other small arms. Formally, the specification is MIL-STD-1913, "“Dimensioning Of Accessory Mounting Rail For Small Arms Weapons”, developed at the Picatinny Arsenal and issued on 3 February 1995. It is used for accessories such as gunsights, light sources and supplementary handgrips.
There are variants with two, three, or four mounting rails. Some directly-attached accessories are mechanical mounts for classes of other accessories, such as a dovetail for optical sights, or a bayonet lug.
Pickatinny rail systems have been widely adopted for sporting arms as well. While recent military rifles are manufactured with the system, it is quite common to find adapter kits that replace the existing grip on a rifle or shotgun with a rail system, and provide a similar rail-mounted grip.
A related commercial accessory rail specification is the "Weaver". In general, Weaver accessories will fit a Picatinny rail, but not vice versa.