Definition of "out-of-band"

out-of-band

adjective
1. (telecommunications) Relating to activity outside of a defined telecommunications frequency band.
  Quotations
  In this additional spectrum, in-band signaling was sent down the wires outside the frequencies used for conversation. Actually, the signals were sent across the 3,500- and 3,700-Hz frequencies. Although these worked and were not in the talk path (out of the band) they were limited in the number of tones that could be sent.
  2001, Regis J. Bates, Donald W. Gregory, Voice and Data Communications Handbook‎, page 841
2. (telecommunications, by extension) Relating to communication on a different channel, or by a different method, from that of the primary communication channel.
3. (computer security) Relating to communication, such as identity verification, via a method other than the primary means of accessing the software.
  Quotations
  One great way of solving the phishing problem is doing out-of-band authentication.
  2007, Simon Willison, Google Tech Talks: The Implications of OpenID‎, archived from the original on 13 May 2008
  Defaulting to chaos still causes a lot of disruption, and would require out-of-band social coordination to resolve, but it places a much larger barrier in front of the attacker, and makes attackers much less confident that they will be able to get away with a clean victory, making them much less motivated to even try to start an attack.
  2020, Vitalik Buterin, “A Philosophy of Blockchain Validation”, in vitalik.ca‎
4. (programming) Referring to a value returned by a function that is not in its natural range of return values, but rather signals an exception.