A ground is the surface on which you paint, usually a coating (such as gesso primer) rather than a support except when the support is paper.
A ground can be any color, although white is the most common. To create a colored ground, color can be added to the primer or a layer of color applied over the priming. (View an example of a painting done on a colored ground.)
An absorbent ground is one that pulls in or absorbs a paint, rather than letting it sit on the surface.
See Also: Painting on a Colored Ground