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. Because of this, the nature of the service specified by this term should always be verified. Source: From Federal Standard 1037C and from MIL-STD-188