Every team that qualifies for the playoffs has someone on its roster who's capable of turning an entire series around if he gets hot or plays up to his highest capabilities.
These players could come off the bench or be starters who hold great responsibility—it doesn't matter. An X-factor is someone who greatly increases his team's chances of winning if he plays well, and hinders his team's chance of success if he flatlines.
It's usually a wild card, but doesn't have to be. Some of the players on this list are All-Stars, but that doesn't mean we know what to expect from them on a game-to-game basis.
If these X-factors get it together in a seven-game series, their teams will be very difficult to take down.