Usually used to refer to an ongoing thing, like a television program or a band's musical career, this term is used retrospectively to refer to the point where the TV Show (or whatever) peaked and everything after that it began to atrophy.
Supposedly the term arose from an episode of Happy Days when Fonzie jumped over a shark (in a shark tank) with his motorcyle.
See also Boiling Point and Wingman.
This bit of jargon has been viewed 9,189 times.