A heightmap is a raster image. Each pixel of the map stores values, such as surface elevation data. Blender uses height maps in materials through image textures as a way of faking (bump map) or producing (displacement) displacement using the material, along with other uses. Use this tag for questions about the use of heightmaps in Blender.

Wikipedia describes a Heightmap thusly:

In computer graphics, a heightmap or heightfield is a raster image used mainly as Discrete Global Grid in secondary elevation modeling. Each pixel stores values, such as surface elevation data, for display in 3D computer graphics. A heightmap can be used in bump mapping to calculate where this 3D data would create shadow in a material, in displacement mapping to displace the actual geometric position of points over the textured surface, or for terrain where the heightmap is converted into a 3D mesh.

The Blender Bump node can create the appearance of displacement using an Image Texture as a heightmap. Heightmaps may also be used to directly drive the Displacement node to create actual displacement of the vertices in an object. The Displace Modifier can also use a heightmap to create displacement outside of a material.

Use this tag for questions about any of the uses of heightmaps in Blender.