One of the basic building blocks of plot, a Plot Twist is a sudden, unexpected change in the fortunes or situations of the characters, setting, or plot.
Plot Twists are usually based on the assumption that there is something going on that we, the readers/viewers/players, don't know about; if we had known about it, it would hardly be surprising. When it is [[TheReveal revealed]] to us, we are surprised and shocked. This includes hidden aspects of particular characters' backstories or their personalities ("I never would have thought Alice could kick Bob's head into the locker!")
Sometimes even when we ''do'' have all the information, the twist can come as a shock due to a sudden, unforeseen action by other actors in the plot, the setting or place it's occurring in. We could see an action that we'd long predicted but not all its ramifications, so the shock value is still there. Alternatively, while we've seen characters' emotions, we may not always be clued into the extent of their thoughts, so a particular action may still come as a shock.
A good Plot Twist relies on the viewer not to figure it out in advance. When the viewer already knows, due to it being telegraphed ahead or them being [[ItWasHisSled previously spoiled]], the twist is often a letdown. On the other hand, if a plot twist comes ''too'' out of nowhere, it can leave the viewer feeling cheated.
Note that Plot Twists refer to twists in the ''overall plot'', and generally have lasting effects. An individual scene, such as a fight scene, can have several reversals in the fates of the combatants, but none of them can be called a Plot Twist. If the twist is of minor significance to the overall plot, it does not fall into this category.
Contrast DramaticIrony, which is where the ''audience'' knows something that one or more ''characters'' do not.
* MaskOfSanity (when the character is revealed to be crazy later instead of from the start)