back channel

Short definition from Wiktionary

Noun

1. 1
   The smaller of two channels in a river that diverge to form an island.
2. 2
   A secondary channel of activity, typically in parallel with primary channels and somewhat independent of them.
3. 3
   An unofficial communications channel used to make informal or subversive negotiations.
4. 4
   A real-time online discussion that occurs simultaneously with a verbal presentation.
5. 5
   A gesture or short prompt by which a listener invites a speaker to continue.

Verb

1. 1
   Alternative form of backchannel.