1. In telephony, a signal, usually consisting of an audio tone interrupted at a slow rate, provided to a caller to indicate that the called-party instrument is receiving a ringing signal.
Note: This signal may be generated by the called-party servicing switch or by the calling-party switch.
2. A ringing signal returned to a caller to indicate that one of the types of delayed automatic calling is now ringing the called party.
Source: from Federal Standard 1037C and from MIL-STD-188