1. In radio communications systems, pertaining to a station that is completely shut down, i.e., that is not transmitting any signal, not even an unmodulated carrier.
2. In a radio station, pertaining to a particular source of modulation, such as a specific microphone, that is disconnected, i.e., is no longer capable of modulating the carrier.
Note: The carrier may continue unmodulated or it may be modulated by another signal source.
Source: from Federal Standard 1037C