In telecommunications

A back-channel in an asymmetric telecommunications system is typically a low-speed or less-than-optimal transmission channel opposite to the main channel's direction.

An example of this is in ADSL where "A" stands for "asymmetric", and the channel from the subscriber to the supplier (ISP, is slower and may be of less capacity than the channel from the supplier to the subscriber.

In many cases, the back-channel is used mostly for acknowlegements of the validity of the forward-channel's data (i.e. that the forward-channel's data passes validity tests of some sort).

In diplomacy

A back-channel in the language of diplomacy is an unofficial channel of communication between states or other political entities, used to supplement official channels, often for the purposes of discussing highly sensitive policy issues.

In linguistics

In linguistics, back-channeling is the practice of giving positive comments, such as "uh-huh" or "yes" to the other speaker, to encourage further talk or to confirm that one is listening.

Back-channeling is very common in Japanese, and is said to be used more often by female speakers of English than male ones. It is also an important part of dialogue in American Sign Language.