A Quick and Simple Dictionary

out-of-band

1. Adjective.  (context, communications engineering) Out-of-band signaling describes signals that are sent between two parties or two devices that are sent via a path or method different from that of the primary communication between the two parties or devices.
2. Adjective.  (computer security) Out-of-band communication or out-of-band identity verification involves a program or authentication system's challenge and response to a user via a method other than the primary means of accessing the software. For example, a web application might request verification of a user's identity by sending that user an IM, rather than by presenting a web form for username and password entry.