Back channel

secret or indirect means of communication; return link in a two-way data channel, channel which carries data in the direction opposite that of the primary channel (Telecommunications)

In telecommunications A back-channel (also reverse channel or return channel) is typically a low-speed, or less-than-optimal, transmission channel in the opposite direction to the main channel.See: Return channel
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Noun
1. 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"
(hypernym) channel, communication channel, line

Adjective
1. via a back channel; "thefailure of back-channel negotiations"
(pertainym) back channel