In telecommunication, the term simplex circuit has the following meanings:

1. A circuit that provides transmission in one direction only.

2. Deprecated definition: A circuit using ground return and affording communication in either direction, but in only one direction at a time.

Note: The above two definitions are contradictory; however, both are in common use. The user is cautioned to verify the nature of the service specified by this term.

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