The phrase deus ex machina (Latin IPA: [ˈdeːus eks ˈmaːkʰina] (literally "god out of a machine") describes an unexpected, artificial, or improbable character, device, or event introduced suddenly in a work of fiction or drama to resolve a situation or untangle a plot (e.g. an angel suddenly appearing to solve problems).

Linguistic considerations
The phrase has been extended to refer to any resolution to a story that does not pay due regard to the story's internal logic and is so unlikely that it challenges suspension of disbelief, allowing the author to conclude the story with an unlikely, though more palatable, ending.
In modern terms the deus ex machina has also come to describe a being, object or event that suddenly appears and solves a seemingly insoluble difficulty, where the author has "painted the characters into a corner" that they can't easily be extricated from (e.g. the cavalry unexpectedly coming to the rescue, or James Bond using a gadget that just so happens to be perfectly suited to the needs of the situation).
Other examples are seen in Dante Alighieri's Inferno when a mysterious personage (variously identified) "sent from Heaven" clears the path of fallen angels and opens the gates of Dis for Dante and Virgil to pass; in H.G. Wells' War of the Worlds where the Martians suddenly succumb to common viruses. The device is a type of twist ending.
The notion of deus ex machina can also be applied to a revelation within a story that causes seemingly unrelated sequences of events to be joined together. Thus the unexpected and timely intervention is aimed at the meaning of the story rather than a physical event in the plot. This may more accurately be described as a plot twist.