What is "back channel"

n. 1 The smaller of two channels in a river that diverge to form an island 2 An unofficial communications channel used to make informal or subversive negotiations 3 (context linguistics English) A gesture or short prompt by which a listener invites a speaker to continue.

n. an alternative to the regular channels of communication that is used when agreements must be made secretly (especially in diplomacy or government); "they negotiated via a back channel"