A raster image which can depict a 3D environment through mapping out the height at every point. Can be thought of as a mechanically consumeable form of topographical map.
A heightmap is a very efficient method of storing data on 3D structures such as terrain. The value of each point corresponds to the height, allowing elements such as complex mountain ranges to be stored in a mere 2D graphic. It can be pixel-smooth for full 3D games, or it could be on a per-tile basis when dealing with a tile-based map.
Use this tag when your question deals with the construction, storage, or implementation of a heightmap in a game.