Specular mapping
A texture that dials specular highlight strength per texel, letting one surface be wet-shiny in the cracks and dry-matte on the ridges.
Where a plain colour texture (albedo) only says what colour a surface is, a specular map controls how much and how sharply it reflects. Wet cobblestone, scratched sheet metal or a sweaty forehead get highlights that follow the material rather than sitting evenly across the whole surface. The channel often carries glossiness too, deciding how tight or smeared the reflection is.
The technique was standard in the fixed-function and early-shader era — Half-Life 2 (2004) leaned on it heavily. In physically based rendering the idea has split into roughness and metalness maps, which describe the same thing in measurable terms.