From Alfred Hitchcock Wiki

A MacGuffin is a plot device that motivates the characters and advances the story, but has little other relevance to the story itself.

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.
(Alfred Hitchcock, 1966 interview with François Truffaut)

• Blackmail (1929) - the glove
• Number Seventeen (1932) - a jeweled necklace
• The Man Who Knew Too Much (1934) - the assassination
• The 39 Steps (1935) - what is "The 39 Steps"?
• Young and Innocent (1937) - the coat and the missing belt
• The Lady Vanishes (1938) - the tune that is a secret code
• Foreign Correspondent (1940) - Clause 27
• Notorious (1946) - the uranium ore
• Strangers on a Train (1951) - Guy's lighter
• Rear Window (1954) - what is buried in the garden? and possibly, Mrs. Thorwald herself
• North by Northwest (1959) - George Kaplan; the secret microfilm
• Psycho (1960) - $40,000 and the whereabouts of Marion Crane
• The Birds (1963) - what is causing the birds to attack?
• Torn Curtain (1966) - the secret formula known by Prof. Lindt
• Topaz (1969) - the spy ring
• Family Plot (1976) - the diamonds