Irradiance map
A precomputed texture that stores, for every direction, how much diffuse light arrives from it, so indirect lighting can be looked up instead of computed in real time.
Diffuse global illumination — the light that bounces off walls and sky before reaching an object — is expensive to compute per frame. An irradiance map moves the work offline: you integrate incoming light over the whole hemisphere for each direction once and store the result, usually as a small cubemap or encoded in spherical harmonics.
At render time a single lookup along the surface normal gives its diffuse contribution. Because irradiance varies smoothly, no high resolution is needed — a handful of coefficients goes surprisingly far. The technique is the backbone of image-based lighting and light probes; for glossy surfaces it is paired with a separate, sharper reflection map. Static maps of course lock the lighting in place, so dynamic scenes have to refresh them.