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.

Slide stop in its "up" position, locking the slide of this Glock pistol in its "back" position. Pressing down the switch can release the lock and cause the slide to spring forward but this may accelerate wear and should not be used in a defensive situation.

Slide stop (with mechanism exposed, larger circle) and corresponding notch in slide, which catches the slide stop

...
Wikipedia