Ambient occlusion

Ambient occlusion is a technique used in computer graphics to calculate the extent that any given point in a three-dimensional scene is illuminated by ambient light. For example, in an outdoor scene, the sky is considered the source of ambient light. So, any given point in the scene may be assigned a value according to how exposed it is to the sky. More specifically, when traveling through a tunnel, the entrance is more exposed to the sky's ambient light, but deeper parts of the tunnel are darker or more occluded.

3D, Programming terms, Radiosity, Ray tracing