Ground

1. An electrical connection to earth through an earth-electrode subsystem.

2. In an electrical circuit, a common return[?] path that usually (a) is connected to an earth-electrode subsystem and (b) is extended throughout a facility via a facility ground system consisting of the signal reference subsystem[?], the fault protection subsystem[?], and the lightning protection subsystem[?].

3. In an electrical circuit, a common return path that (a) may not necessarily be connected to earth and (b) is the zero voltage reference level for the equipment or system.

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