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)
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.
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.