A splat map is a texture which controls blending of multiple textures (or other values) across a model.
Splat maps are commonly used for terrains in games.
Each color channel of the splat map is a mask for a different texture. Usually four textures are blended, using the red, green, blue, and alpha channels of a splat map.
See also MultiTexture#Texture_Splatting.
A splat map is used to blend terrain textures. See UDN - Three - Terrain Advanced Textures.