TALLY LIGHT

In a television studio, a 'tally light' is a small signal-lamp on a television camera or monitor. On a camera, it is usually located just above the lens and indicates, for the benefit of the actors being filmed, that the camera is 'live' - ie: its signal is being recorded or transmitted at that moment. Tally lights are typically red, although some cameras and video-switchers accept a preview tally signal, which is typically green.
For television productions with more than one camera, the tally lights are generally illuminated automatically by a switcher or video router. The switcher keeps track of which cameras are on-air and closes the appropriate electrical contacts, which are generally wired to the camera control unit. If more than one camera is on-air simultaneously (as in the case of a dissolve), the tally lights for both cameras will be briefly lit.