Usually a single curse word or indecency during a live TV or radio broadcast. It's the media's version of the "F-Bomb", used by the FCC in assessing fines against broadcasters.
Joe Biden's comment on Health Care Reform after introducing the President ("...it's fucking important...") was a Fleeting Expletive.
Janet Jackson's wardrobe malfunction in the half-time show of Super Bowl XXXVIII was a visual Fleeting Expletive.
1. A gimmick or storyline may be reused freely and safely after a few years of dormancy.
2. The unwritten rule that, after a given number of years, there has been enough turnover in the fanbase that a writer can re-use the same gimmicks and storylines with impunity.
The fleeting demographic rule allows YouTubers to reuse jokes and catchphrases after they have become repetitive.