Ground

In electrical engineering, the term ground has the following meanings:

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 in support of MIL-STD-188