Deus ex machina
Etymology
Latin, "god from the machine," translating the Greek "theos ek mēchanēs." It refers literally to the crane or platform (the "mēchanē") that lowered an actor playing a god onto the stage in ancient Greek drama to sort out the plot.
noun phrase · /DAY-oos eks MAH-kih-nuh/
An improbable, contrived device that abruptly resolves a story’s problem.
What it names
The cheat ending — a sudden rescue, inheritance, or coincidence that untangles the plot without being earned, and a useful charge to level at lazy resolutions.
Example
The heroes are trapped with no way out — then a deus ex machina arrives in the form of an eagle no one mentioned before.
Related idea
The opposite of good craft as captured by Chekhov’s gun: a deus ex machina resolves things with something unplanted, whereas Chekhov’s principle demands the resolution be set up in advance.