a term to describe a moment when somethin that was once great has reached a point where it will now decline in quality and popularity.

Origin of this phrase comes from a Happy Days episode where the Fonz jumped a shark on waterskis. Thus was labeled the lowest point of the show.

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.

Refers to the specific moment when a TV show goes downhill. From the Happy Days episode where Fonzie overcomes his fear of sharks by jumping over one while wearing water skis.

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.

when something stops getting better and starts getting worse

The precise moment when you know a tv show, rock band, or actor has gone downhill.

to reach the point where the poupularity of a show, movie, musician, or any other pop culture icon declines in popularity (refers to an episode of The 1960's TV show, "Happy Days" where Fonz jumps over a shark while water skiing, see also jump the couch)