MacGuffin

“The main thing I’ve learned over the years is that the MacGuffin is nothing. I’m convinced of this, but I find it very difficult to prove it to others.” –Alfred Hitchcock

Other wise known as McGuffin or maguffin, a MacGuffin is a plot device that is usually given little or no narrative explanation. It can be in the form of a goal, desired object, or other kind of motivator, and can be either a person, object or place. Sometimes, you might not even notice what the MacGuffin is in a film, but it's always there, and it's the driving force behind the main character's actions in a film.

Alfred Hitchcock is famous for using this term throughout his films.

MacGuffin use in Hitchcock films

Vertigo: Carlotta Valdes' character never appears in the film, and her death isn't important at all to the audience. However, because she is apparently possessing Madeleie Elster, it starts the case for Scottie to investigate and drives the entire film's plot. Some also have noted that the necklace or Scottie's vertigo is also a MacGuffin.

The 39 Steps: Mr. Memory's memorization of plans for an airplane engine. 

Rear Window: the "murder" committed by Lars Thorwald.

Notorious: the uranium ore that is in all the vintage wine bottles.

The Birds: the reason why the birds are attacking everyone.

Psycho: the $40,000 in the envelope. 

His famous method has also inspired other films to use this method in their films. Some examples are:

Pulp Fiction: the briefcase

Avatar: the mineral unobtainium

Mission: Impossible III: the rabbit's foot

Casablanca: the letters of transit

Citizen Kane: the rosebud