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.