Deus ex machina is a sudden, improbable plot resolution from an external force, character, or event.
The phrase is Latin for 'god from the machine' and refers to a piece of theatrical equipment in ancient Greek drama: a crane that lowered an actor playing a deity onto the stage to resolve an otherwise unresolvable plot. Euripides was famous for using it, and even his contemporaries thought he over-relied on it. Aristotle in the 'Poetics' singled it out as a flaw — a plot should resolve from its own internal logic, not from a divine shortcut. The term has become shorthand for any contrivance: the cavalry arriving at the last second, the inheritance discovered just in time, the killer slipping on a banana peel. Used self-consciously, as in 'Monty Python and the Holy Grail', it becomes comedy; used unselfconsciously, it remains the most reliable signal of weak storytelling.