|Definition - The responses you make during a conversation that let your interlocutor know that you are listening to them.|
I see, Uh-huh, and OK
Quotation - "Back-channel signals are verbal and non-verbal signals inserted (reception, support and stimulation) within an interaction by the receiver without interrupting the speaker's discourse. They signify the attentiveness of the receiver (Kerbrat-Orrechionni, 1990)."
(Source: Parisot, A.-M. et S. Villeneuve (2005) "The description of back-channelling for Quebec sign language (LSQ): A pragmatic tool used in interpreter training", International Society for Language Studies 2005, Montréal. )