Wikipedia
Relative to audio engineering, a mix-minus is a particular setup of a mixing console or matrix mixer, such that the output to a certain device contains everything except the input from that device. Mix-minus, which is technically accomplished via a wiring and patching configuration, prevents echoes or feedback from reverberating or howling and squealing through the broadcast or sound reinforcement system.
A mix-minus signal is sometimes also known as a 'clean feed' or a 'Select Audio Return' (SAR), it can be written as N-1. Some broadcast mixing desks, notably those designed in house by the BBC, maintain a separate mix bus for 'clean feeds'. This is technically different, and arguably superior to most implementations of mix-minus, but the end result is the same.