"Jumping the shark" is a colloquialism used by TV critics and fans to denote the point in a TV or movie series when the characters or plot veer into a ridiculous, out-of-the-ordinary storyline.
Shows that have "jumped the shark" are deemed to have passed their peak, since they have undergone too many changes to retain their original appeal, and after this point fans often notice a decline in quality.
The term refers to a scene in a 1977 episode of Happy Days when Arthur "Fonzie" Fonzarelli literally jumps over a shark while water skiing. The scene was considered so preposterous that many believed it to be an attempt at reviving the declining ratings of the flagging show. Ironically, not only was Happy Days reflecting the superstardom of real-life shark-jumper Evel Knievel in the episode, but the series was wildly successful in 1977. Happy Days was the second most popular show on television that year, second only to spin-off, Laverne & Shirley.
Jump-the-shark moments may be viewed as a desperate and futile attempt to keep a series fresh in declining ratings. In other cases the departure or replacement of a main cast member or character or a significant change in setting changes a critical dynamic of the show. These changes are often attempts to attract their fans' waning attention with over-the-top statements or increasingly overt appeals to sex or violence.
The term has also evolved to describe other areas of pop culture including movie series, musicians, actors or authors for whom a drastic change was seen as the beginning of the end or marking the moment the subject is "past its peak." When referring to celebrities, the related term "jumping the couch" is often used if the moment is a personal act of "going off the deep end".
Even before "jumping the shark" was employed as a pop culture term, the episode in question was cited many times as an example of what can happen to otherwise high-quality shows when they stay on the air too long in the face of waning interest. The infamous scene was seen by many as betraying the Happy Days' 1950s setting by cashing in on 1970s fads of Evel Knievel and Jaws.
Producer Garry Marshall later admitted that he knew the show had lost something as the crew prepared to shoot the scene. As Marshall pointed out in the reunion special that aired in 2005, however, Happy Days went on to produce approximately 100 more episodes after the "shark" episode. During the special, question, Marshall introduced the notorious clip and noted that the show had inspired the term.
The first public use of the phrase as a direct metaphor is reported to have been in 1997, when the jumptheshark.com website was launched by Jon Hein. According to the site, the phrase was first coined by Hein's college roommate, Sean Connolly in 1985. The term first appeared in print in the May 29, 1998, Jerusalem Post newspaper article, "It's All Downhill," written by Jeff Abramowitz.
The phrase has been used more recently outside the realm of popular culture, representing anything that has reached its peak and has turned mediocre, such as a stock or a sports team.
Arrested Development has a character played by Henry Winkler, who played the Fonz in Happy Days. In the episode "Motherboy XXX", while conversing with other characters on a dock, he remarks, "I missed breakfast, so I’m on my way to Burger King," and then hops over a shark that's in his path.
That '70s Show had an episode in which Fez imagines jumping over a shark, thinking how cool it would be to be the Fonz. Hyde comments that not only is it the worst idea ever, but that it also was the worst moment in television history. Fez then says he never really watched the show after that episode. In another episode, Eric asks Pastor Dave how cool Jesus is compared to Fonzie, and asks if he can jump over a shark. The series often utilized 1990s points of view rather than reflect the actual 1970s view where the episode was a huge ratings success.
Mad TV reenacted a skit in which the infamous "jump the shark" episode was partially redone in mock Spanish, featuring dialogue such as Laverne saying "Aww, Shirl, Fonzie es jumpo el sharko!" (info from Wikipedia)