MacGuffin

MacGuffin: The item around which the action of a story revolves. Termed by, I believe, Alfred Hitchcock, or at least popularized by said filmmaker. Example: the sword "Green Destiny" was the MacGuffin in Crouching Tiger, Hidden Dragon.

Comments

Another classic MacGuffin is the Ring from the Lord of the Rings series.
--BrilliantRain

Other well-known MacGuffins are the Maltese Falcon or the letters of passage from Casablanca. -Ben-San

Although not universal, an important element of the MacGuffin is the idea that it's important just because it is -- not because it necessarily has any utility, per se. The Maltese Falcon is an excellent example of this. -- AntiVehicleRocket

Often, the MacGuffin isn't even explained, like the mysterous contents of the briefcases in Pulp Fiction and Ronin. - Wordman