Emission on a frequency or frequencies immediately outside the necessary bandwidth which results from the modulation process, but excluding spurious emission. [NTIA] [RR]

In encryption, notification using a communication means other than the primary communication means. [After X9.49]

Signaling that uses a portion of the channel bandwidth provided by the transmission medium, e.g., the carrier channel, which portion is above the highest frequency used by, and is denied to, the speech or intelligence path by filters. Note: Out-of-band signaling results in a lowered high-frequency cutoff of the effective available bandwidth. 2. Signaling via a different channel (either FDM or TDM) from that used for the primary information transfer.