back·channel
noun/ˈbæktʃænl/
/ˈbæktʃænl/
- a secret or unusual way of passing information to other people
（传递消息的）秘密渠道，特殊途径 - (linguistics) a sound or sign that somebody makes to show that they are listening to the person who is talking to them
言语反馈示意，反输，附应（以声音或示意作为对说话人的反馈）