Slide Stop
A lever that holds the slide to the rear when the magazine is empty, and that some shooters use to release it.
Also: slide release, slide lock
The slide stop is engaged by the magazine follower when the last round is fired, locking the slide rearward as a visual indicator that the gun is empty. The same lever can be pressed down to release the slide on a freshly inserted magazine — though many trainers prefer racking the slide instead, since slide stop / slide release is one of the longest-running terminology debates in handgun training.