The precise moment when you know a program, band, actor, politician, or other public figure has taken a turn for the worse, gone downhill, become irreversibly bad, is unredeemable, etc.; the moment you realize decay has set in.
Tom Cruise jumped the shark the moment he jumped Oprah's couch.
Original meaning was the point when a television series shows it has run out of ideas and must resort to stunts to retain viewer interest. Derived from a scene in the last years of Happy Days when the Fonz waterskied over a shark. Includes actions such as "it was all a dream" episodes, live episodes, lead actors playing guest characters, and putting entire cast into a parody of some pop cultural event.
It looks like "Will and Grace" decided to jump the shark. They are doing a live episode in black and white where they are all characters from Star Wars. Elton John will play Jabba the Hut who shoots Jack with a "straight ray" gun.
The beginning of the end. Something is said to have "jumped the shark" when it has reached its peak and begun a downhill slide to mediocrity or oblivion. It's said to have been coined by Jon Hein, who has a web site, jumptheshark.com, and now a book detailing examples, especially as applied to TV shows. It supposedly refers to an episode of the TV show "Happy Days" in which Fonzie jumps over a shark on water skis, which Hein believes was the point at which the series had lost its touch and was beginning to grasp at straws.
A TV show's sure "jump-the-shark" sign: the appearance of a "special guest star".