out-of-band

Short definition from Wiktionary

Adjective

1. 1
   Relating to activity outside of a defined telecommunications frequency band.
2. 2
   Relating to communication on a different channel, or by a different method, from that of the primary communication channel.
3. 3
   Relating to communication, such as identity verification, via a method other than the primary means of accessing the software.
4. 4
   Referring to a value returned by a function that is not in its natural range of return values, but rather signals an exception.