racking slide
1) The act of manually pulling back on the slide mechanism of a semiautomatic pistol in order to chamber a round for firing.
Usually this is done after insertion of a fresh magazine to load the first round, but can be done anytime to forcibly remove an existing round or clear a jam (e.g. stovepipe).
2) The act of said slide being forced back during normal operation of the semiautomatic handgun. Not often caught in still photo, but more so in slow-motion video and drawn images for dramatic effect.