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.
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