Player can lean left and right to peek around corners without revealing much of their body, either to avoid being detected or to reduce the hitting area for incoming fire.
Name variations: peeking
The first video game about Leaning was released on March 26, 1994.
In strategy games this is assumed to be automatic when characters move near corners in buildings or behind objects or similar so they lean to the side so their line of sight is no longer blocked. Though in some instances this can be player invoked, such as in turn-based games where leaning/peeking takes action points or similar.
With sufficient sophistication (and context limitation) this turns into cover system.