DEUS EX MACHINA
This means when a problem in a story is abruptly resolved by the introduction of a new concept into the story- a new character, idea, superpower, etc.
This term is constantly abused as referring to anything that saves the day. The shot Luke makes to blow up the Death Star isn't a Deus Ex Machina, becuase it's been built up to the entire movie. Gandalf and the Cavalry saving the day in "The Two Towers" is not a Deus Ex Machina because Gandalf left in act two to go find the characters they met in act one, to come back and save the day.
R2 having rockets could be seen as a Deus Ex Machina. R2 fixing the hyperdrive in "ESB" isn't, because the audience has known for some that R2 knows about the hyperdrive.
This is especially annoying when the meaning is flipped completely, and used to refer to the beginning of a story. I've heard "That the droids end up in the hands of Luke is such a Deus Ex Machina."