Lightmap
A texture that stores precomputed lighting for static geometry, baked once so the runtime needn't recompute it every frame.
Lightmapping precalculates how light — including bounced, indirect light — falls on fixed surfaces and stores the result in a second set of UVs and a texture. At runtime the engine simply multiplies the surface color by its lightmap, getting soft shadows and global illumination at almost no cost. Quake (1996) popularized the technique; it's why old levels with no dynamic lights still look richly shadowed.
The catch is that baked light is frozen: move a wall or a lamp and the lightmap is wrong until it's rebaked, which can take minutes to hours. So lightmaps suit static scenery while dynamic objects use real-time lighting, and the two are blended. Modern engines add light probes and increasingly fall back on real-time GI or ray tracing for fully dynamic worlds.