Jump the shark
Jumping the Shark is the moment when a TV series, game, franchise, or other pop-culture phenomenon signals that its best days are clearly behind it and it has begun a sad descent into shittiness. It was originally a reference to a gimmick that writers concocted in an attempt to attract viewers back to a show whose ratings had been in decline for a while, like when Fonzie literally jumping over a shark to attract viewers back to Happy Days or when The Simpsons did that whole "Who shot Mr. Burns?" event. In recent decades, however, it has come to mean any watershed moment in a franchise's history that is associated with a decline in quality. Examples have included:
1) The creator leaving or being fired from their own show/company (example: Lauren Faust leaving My Little Pony) 2) Characters being killed off or otherwise written off 3) New, annoying, useless characters being added (Scrappy Doo) 4) The two main characters finally hooking up (Bones) 5) Taking the show's entire concept and flushing it down the shitter in favor of something radically different (Red vs. Blue: the Blood Gulch Chronicles) 6) Michael Bay turning an '80s cartoon into a movie 7) Corporate buyouts (Youtube being bought out by Google) 8) Adding wacky sci-fi and fantasy elements to a show that had previously been very grounded in reality (Baywatch Nights, Family Matters) 9) Trying to keep a show going after most of its actors quit and there was already a fucking final episode (Scrubs) 10) The writers finally making it clear that they never had any kind of plan and were just making shit up as they went along despite spending years telling fans that they had a plan (Lost, Battlestar Galactica) 11) Matt Ward writing literally anything
It is sometimes possible for a franchise to jump the shark, stabilize at a certain level of shittiness, and then jump the shark again to become even worse. For example, Magic: the Gathering jumped at least twice: the first time being the 6th Edition rules changes, and the second time being the 8th Edition card frame/border. It has probably jumped another five or six times since then, but we wouldn't know because the last players with any functioning brain cells haven't paid attention since Time Spiral (and even then, it was only for the timeshifted cards). World of Warcraft is also commonly considered to have jumped twice; the first being Cataclysm and the second being Warlords of Draenor, though people whose opinions aren't tainted by nostalgia goggles will usually admit that the Shattering was actually a huge improvement.
It is also possible, however rare, for something to "jump back" and return to a quality level that fans previously thought they'd never see again. An example would be when Apple re-hired Steve Jobs to clean their shit up.