Picatinny Rail
A standardized accessory mounting rail with crosswise slots.
Also: Pic rail, 1913 rail
The Picatinny rail (military spec MIL-STD-1913) is a flat rail with regularly-spaced perpendicular slots. It became the universal interface for mounting optics, lights, lasers, and grips. Its consistent dimensions mean accessories from any maker mount on any rail — a major standardization win.