A MacGuffin is a plot device that holds no meaning or purpose of its own except to advance the story. The term is usually used in films, especially thrillers. For instance, the briefcase in Quentin Tarantino's Pulp Fiction is a MacGuffin (and a nod to Kiss Me Deadly) — the contents are never shown; that section of the plot is not about the briefcase so much as what happens because of it.
The term "MacGuffin" was invented by Alfred Hitchcock, and he made extensive use of MacGuffins in his films. One example of a MacGuffin in Hitchcock's movies is the bottle of "radioactive diamonds" in Notorious: it is the reason the story takes place, but it means nothing. The story could just as easily have used "uranium" (which is in fact what was originally meant as the contents of the bottle) or gold or extraordinary rare wine as the plot device.
Another famous MacGuffin (used by someone other than Hitchcock) is the search for "Rosebud" in the Orson Welles film Citizen Kane. The reporter spends the entire movie searching for the meaning of the word, and during the course of the film he (and the audience) learns all about Kane's life. But the true meaning of "Rosebud" is not revealed until the final shot of the film.
The best Hitchcock MacGuffin is considered by film scholars to be the one used in North by Northwest. In this movie, the MacGuffin is the character of "George Kaplan", who is being chased by the enemy spies. Roger Thornhill (Cary Grant) is mistaken for Kaplan by the spies, and so they chase him instead. Thornhill spends the course of the movie trying to find George Kaplan himself... without realizing that George Kaplan does not even exist. Both the hero and the villains of the movie are chasing nothing more than a puff of hot air, making this a true MacGuffin.
The term "MacGuffin" has been illustrated by some with this story:
"Two gentlemen meet on a train, and the one is struck by the extraordinary package being carried by the other. He asks his companion, 'What is in that unusual package you are carrying there?' The other man replies, 'That is a MacGuffin.' 'What is a MacGuffin?' asks the first. The second says, 'A MacGuffin is a device used for killing leopards in the Scottish highlands.' Naturally the first man says, 'But there are no leopards in the Scottish highlands.' 'Well,' says the second, 'then that's not a MacGuffin, is it?'"
