In voice, video, or data communications, the term available line (or useful line or in service) is a circuit between two points that is ready for service, but is in the idle state. Antonyms include busy line, unavailable, engaged, and out of service.
In facsimile transmission, it is the portion of the scan line that can be specifically used for image signals.
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).