A heightfield (also known as a Digital Elevation Model, or DEM) is a bitmap whose component pixels are meant to be interpreted as elevations. More formally, these values are some displacement along a dimensional axis perpendicular to the bitmap's existing two dimensions. Because color bitmaps (particularly grayscale ones) are often used to store heightfields, there is sometimes confusion over what is meant by a "heightfield file". Daylon Leveller and Terragen documents, for example, are true heightfield files.
Heightfields are also sometimes called terrain objects, although this is not quite as accurate, since many types of objects can be used to model terrain.