A slide is a portion of a firearm or gun which moves forward and rearward in order to operate the gun.
It is most often used with semi-automatic pistols, and refers to the top portion of the handgun (see photo). Usually, the slide contains the sights, extractor, and firing pin. The presence of other parts varies according to the make and model of the firearm.
Some other guns have parts which may be called by the name "slide," also...
most notably, pump-action (also known as slide-action) firearms. But by and large, it almost always refers to pistols.
- Russ Chastain