Wiktionary
n. A change in the direction or expected outcome of the plot of a film, novel, computer role-playing game, or other form of narration.
Wikipedia
A plot twist is a radical change in the expected direction or outcome of the plot of a novel, film, television series, comic, video game, or other work of narrative. It is a common practice in narration used to keep the interest of an audience, usually surprising them with a revelation. Some "twists" are foreshadowed.
When a plot twist happens near the end of a story, especially if it changes one's view of the preceding events, it is known as a surprise ending. Sometimes people use a plot twist to describe a sudden change of a situation in real life. It is often assumed that revealing the existence of a plot twist spoils a film or book, since the majority of the film/book generally builds up to the plot twist; however, at least one study suggests otherwise.
A method used to undermine the expectations of the audience is the false protagonist. It involves presenting a character at the start of the film as the main character, but then disposing of this character, usually killing them a device known as a red herring.
Usage examples of "plot twist".
Images and events, be they a catch phrase, a murdered child's name, even a plot twist, echo across the centuries, as do subtler adumbrations of character, in a psychotic tramp's sufferings and, more significantly, in the duality of Dyer/Hawksmoor.
As for this unexpected plot twist in the Amenaj play, she would watch it, of course, but there was nothing she could do about it.
As she drove back to Pine Grove she worked out the plot twist that had occurred to her.
Once Kirstie proved unavailable, we began to reconsider the plot twist that the heretofore loyal Saavik had betrayed the Federation and engaged in a murder conspiracy.
In a plot twist Clive has used before, we learn Boudicca is actually a man dressed and living as a woman.
In all the best Schmitz stories there is that one extra plot twist at the end, that you weren't expecting.
There's a nice plot twist in the end, and the fascinating subject matter and impressive mythic scope are handled creatively and expertly.