Ambient occlusion
Also said as AO, SSAO, Contact shadow
Soft darkening where surfaces meet: in creases, corners and under objects, where less of the surrounding light can reach. It is a shadow with no particular light causing it.
It stops objects looking as though they float above the floor, and it makes shapes solid even in flat lighting. Overdone, it looks like dirty halos around everything.
Say it
“Add ambient occlusion, baked into the level for corners and SSAO at half resolution for moving characters. Keep the radius to 30 cm so there are no halos.”
Where you’ve seen it
Crysis (2007) is credited as the first game to use screen-space ambient occlusion, a technique developed by Vladimir Kajalin at Crytek. It estimates the effect each frame from the depth of nearby pixels.
Not to be confused with shadow
A shadow comes from one light and has a direction. Ambient occlusion comes from all around, and shows how exposed a spot is.