Deus ex machina

A deus ex machina (lat. IPA: [ˈdeːus eks ˈmaːkʰina], literally "god from the machine") is an ironic plot device in which a surprising or unexpected event occurs in a story's plot, suddenly and completely resolving an otherwise unsolvable conflict. It is "an improbable contrivance in a story characterized by a sudden unexpected solution to a seemingly intractable problem." Neoclassical literary criticism, from Corneille and John Dennis on, took it as a given that one mark of a bad play was the sudden invocation of extraordinary circumstance. Thus, the term "deus ex machina" has come to mean any inferior plot device that expeditiously solves the conflict of a narrative.

Source: http://en.wikipedia.org/wiki/Deus_ex_machina