Story structure
Plot device
Any technique in a narrative used to move the plot forward: the umbrella term over deus ex machina, MacGuffins, red herrings and their kin.
A plot device (or plot mechanism) is "any technique in a narrative used to move the plot forward." The article's catalogue shows how wide the umbrella runs: a deus ex machina resolves the story from outside it, a MacGuffin motivates the chase while mattering little in itself, a love triangle sets three characters against each other, a red herring misleads, a quibble turns on the literal wording of an agreement as in The Merchant of Venice, and a plot voucher is an object handed to the hero early that a later obstacle will require. Flashbacks, Chekhov's gun and cliffhangers belong to the same extended family.
For a writer the term is a filing cabinet rather than a rule: naming the device is the start of judging whether it is earned. Every entry in the drawer faces the same test, does it grow from the story's own cause and effect, or does it crank the plot from outside?