A MacGuffin, every one of them *
Clockwise from the top left: The eponymous bird from The Maltese Falcon; the "briefcase full of money" used in innumerable thrillers over the decades; Kate's toy plane, from Lost; the crystal shard, from The Dark Crystal; "The Winslow", from Phil Foglio's Buck Godot; the true Grail, from Indiana Jones and the Last Crusade.
"In crook stories it is almost always the necklace,
and in spy stories it is most always the papers."
— Alfred Hitchcock
MacGuffin (a.k.a. McGuffin or maguffin) is a term for a motivating element in a story that is used to drive the plot. It actually serves no further purpose. It won't pop up again later, it won't explain the ending, it won't actually do anything except possibly distract you while you try to figure out its significance. In some cases, it won't even be shown. It is usually a mysterious package/artifact/superweapon that everyone in the story is chasing.
To determine if a thing is a MacGuffin, check to see if it is interchangeable. For example, in a caper story the MacGuffin could be either the Mona Lisa or the Hope diamond, it makes no difference which. The rest of the story (i.e. it being stolen) would be exactly the same. It doesn't matter which it is, it is only necessary for the characters to want it.
Another common MacGuffin story setup can be summarized as "Quickly! We must find X before they do!".
The term was popularised by Alfred Hitchcock, who actually credited one of his screenwriters, Angus McPhail, with the creation of this concept and the name for it, citing a particular school-boy joke:
A man is riding on a train when a second gentleman gets on and sits down across from him. The first man notices the second is holding an oddly shaped package.
"What is that?" the first man asks.
"A MacGuffin, a tool used to hunt lions in the Scottish highlands."
"But there are no lions in the Scottish highlands," says the first man.
"Well then," says the other, "That's no MacGuffin".
(In other words, please don't ask. Or, alternatively, the object is completely devoid of intrinsic meaning; it is whatever the storyteller wants it to be.)
Hitchcock and Angus McPhail were not the first to formulate this concept. Silent-film actress Pearl White starred in cliffhanger serials (most famously "The Perils of Pauline") in which the characters spent most of their screen time chasing each other for possession of a roll of film, or some other doodad. This device occurred so often in Pearl White's serial films that she routinely referred to the coveted object as a "weenie", using the term precisely as Hitchcock would later use "Mac Guffin".
In academic circles, the term The Golden Fleece is sometimes used in place of MacGuffin, after the artifact from Jason and the Argonauts, which makes this trope Older Than Feudalism.
Compare Magnetic Plot Device.
If you want to start arguing that your favourite series most awesome magical thing isn't a MacGuffin, remember that Tropes Are Tools.
•Artifact Of Attraction: If the object itself is relatively innocuous or irrelevant.
•Artifact Of Doom: When the Mac Guffin doubles as the main antagonist.
•Briefcase Full Of Money: If money isn't really spent during the course of plot, it's only a stock objective.
•Clingy MacGuffin: Its most important attribute is that the person who has it has difficulty getting rid of it.
•Dismantled MacGuffin: The Mac Guffin is split into several parts in different places. Plot coupons are most often this type of Mac Guffin.
•Gray's Sports Almanac: An otherwise unimportant item from the future that, if left in the past during time travel, will have serious consequences.
•Hostage For MacGuffin: The heroes have the Mac Guffin. The Villain has a hostage and wants the Mac Guffin. Trade you?
•MacGuffin Escort Mission: The good guys get the Mac Guffin early on. The rest of the story is about them transporting it somewhere else without losing it.
•MacGuffin Location: The Mac Guffin isn't a thing or a person, it's a place.
•Memento MacGuffin: A MacGuffin that holds sentimental value to one or more characters.
•Mineral MacGuffin: A gem, a jewel, or a rock of some type that holds great power; in spite of the name, may or may not be an actual Mac Guffin.
•No MacGuffin No Winner: Neither side has the Mac Guffin in the end. It's been destroyed, lost, or discovered to be fake.
•Pirate Booty: Older than the Briefcase Full Of Money, and even more likely to be stolen.
•Unholy Holy Sword: The Heroes have something (a sword, usually, but not always) that they want to use for good. But it's intrinsically evil..
See also Its The Journey That Counts, Your Princess Is In Another Castle, All That Glitters and Magic Feather.
This is cross-posted from tv tropes