Height Map
From Ogre Wiki
Height Map - A two-dimensional, generally 8-bit greyscale bitmap that is used to represent a three-dimensional mesh, generally a terrain. The value of each pixel in the map is translated into the height for a vertex or series of vertices when constructing a mesh. Normally, black is "0" and white is "255" and the shades of grey in between are heights between 0 and 255. Also 16-bit grayscale bitmaps are possible for finer differentiation. Then white is 65,536.
Example for a terrain (texture + height map):


