1. In voice, video, or data communications, a circuit between two points that is ready for service, but is in the idle state[?].
2. In facsimile transmission, the portion of the scanning line[?] that can be specifically used for image signals. Synonym useful line.
Source: from Federal Standard 1037C and from MIL-STD-188
wikipedia.org dumped 2003-03-17 with terodump