1 Adam: What myth are we busting today, Jamie?
2 Jamie: "Jumping the shark." This myth says that a popular series starts to go downhill when a character jumps over a shark.
3 Adam: And where are we going to find someone stupidly reckless enough to test that out?
4 {scene change: in a house}
4 Steve: {on the phone} Over a shark, you say? Crikey! I'll do it!

Jumping the shark is...

Oh heck, go read it for yourself. That Wikipedia article is far too long and detailed to recapitulate here. In fact, far too long and detailed for any reasonable definition of sane...

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2014-12-31 Rerun commentary: That Wikipedia article has since been trimmed down a lot. Here's what it looked like on the day this comic first appeared. It was nearly three times the size of the current article, and full of various examples and references.

For a more entertaining and exhaustive treatment of the subject, nowadays you can refer to TV Tropes.