h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

The best definition of MacGuffin I've heard (and the one I go by) is something that motivates the characters in the film, who usually pursue it, and yet could be literally anything for the purposes of the plot (ie it is never utilised in any way). For this reason, I tend to dismiss R2-D2 (Star Wars), the Ark of the Covenant (Raiders) and the One Ring (LotR).

Apparently, the way to identify a MacGuffin is to apply the "wristwatch test" - take the object from the film and, in your imagination, replace it with an ordinary wristwatch. Does the film still work? Examples that pass the test are the Lecter Device in "From Russia With Love", the suitcase in "Pulp Fiction" and the A-TAC from "For Your Eyes Only". Of course, it isn't realistic that people would pursue a wristwatch in these films but as far as the plots and turns of events go they might as well be a wristwatch because they don't do anything other than give the characters something to pursue - true MacGuffins.