back channel

Noun

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