Ambience
Also said as Ambient sound, Atmos, Room tone, Soundscape
The constant quiet sound of a place: a low bed that never stops, plus small sounds scattered at random around you. Nobody notices it until it stops.
Without it, a world feels like an empty set, however good it looks. Ambience sets mood and place before anything happens, and it costs far less than music.
Say it
“Give the cave a bed of 170 Hz rumble that never loops audibly, and a random drip every one to three seconds, panned somewhere different each time.”
Where you’ve seen it
Limbo's sound, by Martin Stig Andersen, works as ambient, processed sound of the world more than as tunes. For Inside he went further, playing synth sounds through a human skull with transducers and contact microphones and recording what came out.
Not to be confused with music
Ambience belongs to the place and has no tune or beat. Some of the best game scores blur the line on purpose.