Filmmaker Alfred Hitchcock is generally credited with coining the term "MacGuffin" to describe an element used to drive the plot of the story along. Since then the term has been latched onto by many in literature, film, theatre, etc.
Most of the time writers make sure the MacGuffin is at least usually defined in some manner during the story and is usually disguised as well so that, while it is used to manipulate the audience through the actions of the characters or events on the screen, most audiences don't pay that much attention to the manipulation itself.
There are exceptions, though. I was in a bit of a snit over James Cameron's use of "Unobtanium" in the movie Avatar. It's one thing to use a MacGuffin, it's quite another to shout at the audience THIS IS A MACGUFFIN!!!
But that aside, the fact of the matter is that MacGuffins are everywhere. When I was much younger Garfield creator Jim Davis had another comic strip called U.S. Acres. Not as popular as his Garfield work, the U.S. Acrescharacters did, eventually, feature in a number of animated shorts. The shorts often spoofed things like James Bond and superheroes and a running gag throughout those stories was to spoof the idea of a MacGuffin by creating a nonsensical object they referred to as "A Thermonuclear Exploding Grelbin Device". As you can tell by the fact that I still remember this today this idea struck me as funny and I have come to prefer "grelbin" to MacGuffin. Therefore, throughout my posts you are likely to see me refer to any MacGuffin-like object or device as a grelbin instead. Because, let's face it, a thermonuclear exploding grellbin device sounds much cooler, doesn't it?