A special texture that can be added to a textured brush to give signage to the brush or variety to the texture tiling.
They are usually, a 256-colour greyscale that is stored in "decals.wad" file. Custom decals to be added to the map should be placed in a custom WAD file with a program like Wally and then be merged into the final .bsp file with the CSG compiler's -wadinclude parameter.
Check out SlayerA's
tutorial on Decals for more info.