In telecommunication, a vox (short for 'voice operated switch') is an acoustoelectric transducer and a keying relay connected so that the keying relay is actuated when sound, or voice, energy above a certain threshold is sensed by the transducer.
Note: A vox is used to eliminate the need for push-to-talk operation of a transmitter by using voice energy to turn on the transmitter. Synonyms voice operated relay circuit , voice operated transmit.
Source: from Federal Standard 1037C and from MIL-STD-188
Vox is also the title of a novel by Nicholson Baker; see the Wikipedia article on this author for discussion.