Texture splatting
Blending several tiling textures across terrain using a control map that says how much of each shows at every point.
Rather than one giant unique texture for a whole landscape, texture splatting tiles a few materials — grass, rock, sand, snow — and blends them per pixel according to a "splat map" (also called a blend or weight map), often packing one material's weight into each color channel. Where the map is green you see grass, where it fades into the red channel you see rock, with smooth transitions in between.
It keeps texture memory small while covering large terrain, and artists paint the blend directly onto the heightmap's surface. The cost is several texture fetches per pixel; large material counts push the blending into shaders or virtual-texture systems. Lightmaps and decals are layered on top for shadows and detail.