Story structure
MacGuffin
A plot object or event that motivates the characters while being unimportant in itself; Hitchcock called it 'nothing at all'.
A MacGuffin is "an object, device, or event that is necessary to the plot and the motivation of the characters, but insignificant, unimportant, or irrelevant in itself." The screenwriter Angus MacPhail coined the name and Alfred Hitchcock popularized it. His favorite explanation: two men on a train, one pointing at a package described as "an apparatus for trapping lions in the Scottish Highlands." Since there are no lions in the Highlands, "that's no MacGuffin!" The point: "a MacGuffin is actually nothing at all," the thing the spies are after that the audience does not care about.
Typically introduced in the first act, it then declines in importance and may be forgotten by the end. George Lucas disagreed, arguing the audience should care about it nearly as much as the characters do (R2-D2, the Ark of the Covenant). For a novelist the term marks the gap between what the plot chases and what the story is about, the want versus the need. The statuette in The Maltese Falcon and the briefcase in Pulp Fiction are the classic examples.