Talkback (studio)
The microphone and button that let the engineer in the control room speak straight into the musicians' headphones.
The control room and the live room are acoustically isolated, which is the whole point and also the problem. Talkback is a simple microphone built into the console or control surface, routed to the cue mix by a button. The control room monitors usually dim automatically at the same time, so the voice does not go out, back into the mic and turn into feedback.
In the other direction there is the listen mic: a heavily compressed room microphone that lets the engineer hear what the musicians are saying between takes. Its most famous use is the gated drum sound Hugh Padgham and Phil Collins stumbled on in 1980 while recording Peter Gabriel's third album. Its most regretted use is when someone forgets the button is latched down and says what they really thought of the vocal take.