The most important expression in every actor's vocabulary.
In stage speech it translates to "For the life of me, I cannot remember what the hell I'm supposed to say next!"
It is the most often heard word during the rehearsal of any play on the day rehearsals go off book.
It is a magic word as it instantly makes the assistant director to tell the actor whatever he is supposed to say next.
The trick to making the magic work is to say "Line!" in character, that is, as if it were part of the script.
It can (and often does) truly confuse the assistant director whenever the word "line" actually is part of the script. How is the poor assistant director to know whether the actor is saying "line" or "Line!". It happened to me (on the actor's end), and was actually quite annoying when the assistant director constantly assumed I could not remember what I was supposed to say, and started criticising me for it. After a few fighting words he finally promised not to fall to that assumption.
The importance of "Line!" is shown even by an opening sketch of Saturday Night Live in which Steve Martin gets very psyched up about doing a great show tonight, then gets everyone else equally psyched up. The scene culminates with Steve Martin crying out, "Live from New York, it's... Line!"