Spatial audio
How a game engine places sounds in the world, so a footstep seems to come from a particular direction and distance.
The basic model is one listener, usually the camera, and a set of positioned sound emitters. Every frame the engine computes distance and direction: distance drives volume through an attenuation curve, direction drives panning across the speakers, and relative velocity can add Doppler shift. On top of that comes audio occlusion for walls in the way and reverb zones for the character of the room. On headphones, simple panning gives way to HRTF filtering, which can also convey elevation and front/back.
OpenAL (2000) offered a 3D audio API modelled on OpenGL, while on the hardware side Aureal and Creative fought over the market until Aureal went bankrupt in 2000. Most of it now lives in middleware such as FMOD and Wwise, or in Steam Audio, which simulates how sound actually bounces off the geometry.