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1.
in film, a plot device that has no specific meaning or purpose other than to advance the story; any situation that motivates the action of a film either artificially or substantively.

Originally coined by Alfred Hitchcock, based on a story where this device was used in a story set on a Scottish train.

(sometimes called a McGuffin)
The suitcase in Pulp Fiction is a modern example of a MacGuffin.
by soma616 May 04, 2006
 
2.
A plot device that motivates the characters and advance the story, particularly one whose importance is accepted completely by the story's characters, yet from the audience's perspective it might be minimally explained or may test their suspension of disbelief if it is scrutinized. The device, usually an object, is common in films, especially thrillers.
It is important that the audience never actually see the MacGuffin. I dunno why.

The term "MacGuffin" was invented by Alfred Hitchcock; according to the Oxford English Dictionary, he explained the term in a 1939 lecture at Columbia University:

In regard to the tune, we have a name in the studio, and we call it the 'MacGuffin'. It is the mechanical element that usually crops up in any story. In crook stories it is always the necklace and in spy stories it is always the papers.
Interviewed in 1966 by François Truffaut, Hitchcock illustrated the term "MacGuffin" with this story:

It might be a Scottish name, taken from a story about two men in a train. One man says, 'What's that package up there in the baggage rack?' And the other answers, 'Oh that's a McGuffin.' The first one asks 'What's a McGuffin?' 'Well' the other man says, 'It's an apparatus for trapping lions in the Scottish Highlands.' The first man says, 'But there are no lions in the Scottish Highlands,' and the other one answers 'Well, then that's no McGuffin!' So you see, a McGuffin is nothing at all.
by Laser Potato May 20, 2005