The term deus ex machina (appropriately enough, meaning "the god from the machine" in Greek) is used to refer to a plot device in which a surprising or unexpected event occurs in a story's plot, often to resolve flaws or tie up loose ends in the narrative. In classical Greek theatre, this often involved an actor representing a god whose powers saved the day, being suspended above the stage by a crane, hence the origin of the term. However, when used in Transformers, it's mainly a new character coming out of nowhere and kicking everyones ass (often used to promote a new toy); when a dead character comes back for some reason; or gets a new body (again, to sell toys). Sometimes, however, it's just conveniently used accesories.