Ambient occlusion
An approximation of how much of the diffuse surrounding light actually reaches each point — creases, corners and tight crevices darken because nearby geometry blocks them.
The effect adds contact shadows and depth without any real light source being computed, which is why it works as a cheap seasoning on top of the rest of the lighting. Film pipelines baked it offline back in the 1990s; the real-time breakthrough was Crytek's SSAO (screen space ambient occlusion) in Crysis in 2007, which reads only the depth buffer per pixel.
SSAO is coarse and noisy, so later variants like HBAO and GTAO sample normals and horizon angles for cleaner results. All of them read the G-buffer, which makes the technique a natural fit for a deferred pipeline.