Normal mapping
A texture whose RGB values encode surface normals, so a flat surface catches light as if it were detailed — with no extra geometry.
Where bump mapping perturbed the normal with a single height scalar, a normal map carries the whole normal vector per texel, usually in tangent space — hence the characteristic bluish textures, since an untouched normal points straight out (0, 0, 1). Lighting uses the perturbed normal instead of the surface's own, and the detail exists only in the shading, never in the silhouette.
The breakthrough came when studios began baking a high-poly model's detail onto a low-poly one — an idea from Krishnamurthy and Levoy (1996) that games such as Doom 3, Far Cry and Halo 2 turned into standard practice around 2004. In deferred rendering the perturbed normal is written straight into the G-buffer and read by the lighting pass afterward.