Ground plane

In telecommunication, a ground plane is an electrically conductive surface that serves as the near-field reflection point for an antenna.

Note: A ground plane may consist of a natural (e.g., Earth or sea) surface, an artificial surface of opportunity (e.g., the roof of a motor vehicle), or a specially designed artificial surface (e.g., the disc of a discone antenna).

Source: from Federal Standard 1037C and from MIL-STD-188