Slide stop

A slide stop, sometimes referred to as a slide lock or (dubiously) a slide release on a semi-automatic handgun is a function that visually indicates when a handgun has expended all loaded ammunition and facilitates faster re-loading by pulling back the slide to advance the first round of a new magazine.

The various terms relate to the two functions of the component: While it automatically catches the slide (locking it back) after the magazine's last round has been fired, thereby allowing the user to easily release the slide by pulling down on the switch, it also allows the user to purposefully stop or lock the slide back by pressing up on the switch while racking the slide.