1. A single direct transmission channel, e.g., voice-frequency circuit, between two positions of a large switchboard to facilitate the interconnection of other circuits appearing at the respective switchboard positions.
2. Within a technical control facility, a single direct transmission circuit, between positions in a testboard or patch bay, which circuit facilitates testing or patching between the respective positions.
Source: from Federal Standard 1037C and from MIL-STD-188