In a theater or auditorium, the ambient-light things. Sometimes they're
fluorescent -- and take a while to get going; in this case, they just go off. Otherwise, the
stage manager will (at the start of a play) call out "House to half, *pause* house out" -- this dims the house lights, and then turns them out, signalling the
audience that the show's gonna start.
They also come up at
intermission and at the
end of the play, signalling everyone that it is, in fact, over.
This writeup is a part of the (as-of-yet-nonexistant) theater metanode.