"Mcguffin"

In his 1966 interview with director-film critic, Francois Truffaut, Alfred Hitchcock said:

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, "Its 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.

In other words a McGuffin is a term for the device or plot element that catches the viewer's attention or drives the logic of the plot, especially in suspense films. According to Hitchcock, the McGuffin can be ignored as soon as it has served its purpose. Examples are the mistaken identity at the beginning of North by Northwest (1959) and the entire Janet Leigh subplot of Psycho (1960).

Source: http://nextdch.mty.itesm.mx