Normal map
Also said as Bump map, Surface detail map
A texture that stores which way the surface faces at every pixel, as a colour. Lighting reads it, so a flat wall gets the shading of bricks and cracks while the geometry stays flat.
It gives you the look of millions of triangles for the price of a handful. It gives itself away only at the edges, where the outline is still straight.
Say it
“Bake a normal map from the 2 million triangle sculpt onto the 3,000 triangle model. Keep the silhouette correct in the real mesh, and let the normal map do the wrinkles.”
Where you’ve seen it
Doom 3 (2004) built its look on normal maps baked from much denser models, so low-polygon characters and walls catch light as though they are full of detail.
Not to be confused with height map
A height map stores how high a surface is and can actually move geometry. A normal map stores only the direction it faces, so it changes the lighting and nothing else.