The transferring of information using an outside or separate channel. Out of band apparatus is recurrently used to disperse shared secrets with symmetric key encryption, to authorize sensitive information and initialize or otherwise facilitate the operation of cryptography or other security validation of a ssl certificate on a secure server.
Any frequency outside the band used for voice frequencies is known as "out of band." Out-of-band signaling refers to the use of transmission facilities other than the primary channel bandwidth for simple transmission control pulses. OOB is also the use of narrow band filters to place a voice signal on a carrier channel below 3,400 Hz, reserving the 3,400—3,700 Hz band for supervisory signals.