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.