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.
Jeff Green might be the most consequential X-factor in the entire playoffs, and nobody defines the term better.
When he's playing well—attacking the basket in transition, making the other team's best player uncomfortable on defense, knocking down spot-up three-pointers and taking advantage of every mismatch Doc Rivers is able to get him into—the Boston Celtics are a formidable basketball team.
When Green isn't doing those things, his team struggles. He might be the best offensive player Boston has, and if Green isn't scoring the ball, the Celtics fall behind easily.
Since the All-Star break, Green has averaged 17.3 points per game, as opposed to the 10.3 points per game he averaged before it.
If you expected a 22-year-old Lance Stephenson to competently start 72 games for the Indiana Pacers this season, I'd call you a liar.
Stephenson is a strong contender for Most Improved Player of the Year, upping his shooting percentage from 37.6 percent last year to 46.0 percent this year—remarkable given his increase in offensive responsibility.
He's one of the league's most aggressive players in the open court and a primary reason the Pacers are able to pester opposing point guards on the perimeter.
Indiana's biggest weakness is its half-court offense, but if Stephenson is able to wreak havoc in transition before the opposing team can set up its defense, that weakness may not be exploited as often.
J.R. Smith will probably win his first Sixth Man of the Year award in a few weeks, not because he's the NBA's best scorer off the bench (he is), but because he's drastically disassociated himself from all his bad habits.
Smith ended the regular season as a consistent source of efficient production, something he's never done before. In his last 15 games he averaged 23.7 points on 50.6 percent shooting, according to NBA.com/Stats. Bench or no bench, those types of numbers are All-NBA worthy.
If he can keep it up and maintain this type of offensive play, the New York Knicks will be an incredibly difficult team to beat.
When it became clear Derrick Rose wouldn't be a major factor during the 2013 regular season, the Chicago Bulls signed Nate Robinson, a score-first sparkplug who could come in and provide the team with an offensive punch.
This season, he's scored at least 22 points eight times, and in only one of those games did he shoot below 50 percent from the floor, according to NBA.com/Stats.
When Robinson crosses half court with the ball in his hands, he immediately looks at the basket, seeing if he has space to pull up for three or launch his rock-solid body into the paint.
Most of the time, his decisions aren't good ones until the ball falls through the net, but with Chicago's offense ranking as one of the league's worst all season, the team needs him to score or it won't have a chance.
Miami is too dominant to need an X-factor, but Chris Bosh fills this space for being the most overlooked star in basketball.
Bosh shot an incredible 52.9 percent on jump shots between 16 feet and the three-point line this season, according to Basketball-Reference.com. If that number stays steady throughout the playoffs, the Heat may not lose a game.
If Bosh begins to misfire, things could get very interesting.
A few years ago, Deron Williams was widely considered to be either the NBA's best point guard, or 1B to Chris Paul's 1A.
That talk has quieted down since, with Williams battling ankle and wrist injuries after being traded from the Utah Jazz to the Brooklyn Nets. But in recent weeks, Williams has regained his superstar form.
Since the All-Star break, he's averaged 22.9 points on 48.1-percent shooting from the floor (42.0-percent shooting on almost seven attempts from behind the three-point line), eight assists and the Nets have outscored their opponents by 7.3 points per 100 possessions with him on the court, per NBA.com/Stats.
If the Nets have arguably the best point guard in the league throughout the playoffs, they could seriously make some noise.
Monta Ellis doesn't see a difference between him and Dwyane Wade, which is obviously silly. But that doesn't mean he isn't an extreme talent on the offensive end.
Ellis can score in bunches. Since the All-Star break, he's scored 20.6 points per game on 44.0-percent shooting (that scoring number leapt to 22.6 in March).
He isn't a threat from distance (he's shooting 28.7 percent from behind the three-point line on four attempts per game), but Ellis is one of the most capable rim-attacking guards in the league.
Everyone expects the Miami Heat to roll over Ellis' Milwaukee Bucks, but if he can somehow become an efficient threat every time he touches the ball, maybe the series goes longer than four games.
Even though his team's yet to actually win anything meaningful, we all know about Jeff Teague in the playoffs.
As the third option (fourth, once Al Horford returned) on an Atlanta Hawks team that had a home-court advantage against the Boston Celtics last season, Teague averaged 14.0 points on 41.2-percent shooting behind the arc.
This year, he enters the playoffs looking for a new contract, and if he wants to get paid like his colleagues—Ty Lawson, Jrue Holiday, Stephen Curry—there's a great chance we see him soar even higher.
Last week, I was discussing Houston's chances of winning a first-round matchup with a friend, and the conversation revolved entirely around Omer Asik needing to play like the best defensive player in the world.
That's really the only way the Rockets can win, isn't it?
Asik has the ability to influence a basketball game's outcome without ever scoring. Going up against the Oklahoma City Thunder, he can swallow more rebounds than anyone, and not worrying about Kendrick Perkins or Nick Collison, he should be able to roam around as a help defender on all drives to the paint.
Asik's stellar defense also leads to Houston being able to run its unstoppably frenetic offense once an opponent misses a shot.
If Asik can prevent the Thunder from living at the free-throw line, the Rockets just might steal a game or two with a barrage of three-pointers and transition baskets.
The Oklahoma City Thunder are similar to the Miami Heat in that everybody expects them to reach the NBA Finals.
A case could be made that Serge Ibaka stepping outside his shell on offense and becoming more than a spot-up threat is the real X-factor for this team, but backup guard Reggie Jackson could steal some of the spotlight, especially if the Thunder once again make the Finals.
He's only 23 years old and put up numbers that weren't anything special in his second season, but Jackson provides athleticism on the perimeter that Kevin Martin can't give them in the wake of James Harden's departure. With him on the court in big moments, Oklahoma City will have multiple penetration threats, and maybe that's what it needs in order to win the whole thing.
Right now, nobody expects anything out of Manu Ginobili. He's scoring the fewest points per game since his rookie year as he hobbles into the playoffs with a strained hamstring.
Ginobili also made just 42.5 percent of his shots from the field this year, which stands as the second-lowest figure of his Hall of Fame-worthy career.
He's always been San Antonio's ace in the hole, but now that he's 35, will Ginobili be able to impact the game like he used to? If he does, the Spurs will be headed to the NBA Finals.
Zach Randolph was an All-Star this season, but he wasn't even the best big man on his own team (that honor is bestowed upon Marc Gasol). Why is he an X-factor?
Because when Randolph is at his best, down low in the post, sticking his butt into defenders and dropping that quick, turnaround, left-handed hook shot, there's nobody in the world who can stop him.
When he isn't at his best, the Memphis Grizzlies don't have enough offense to win a playoff series.
So, on one hand, we have an unstoppable force that could conceivably carry a gritty Memphis team to the NBA Finals, and on the other, there's a total non-athlete unable to get his shot off around the rim, one who can hardly move quick enough to gobble up loose rebounds.
That's an X-factor.
Eric Bledsoe at his best, unleashed for a hypothetical 30 minutes per game, is probably the Los Angeles Clippers' third-best player.
For a point guard, he's above average in several areas, including rebounding, playing perimeter defense and getting to the rim.
He's spent most of this season either injured or handcuffed by his own head coach, but if the shackles are removed during the playoffs, the Clippers will come to battle with one of the most athletic players in a league loaded with freakish athletes.
He's capable of taking a game over, and not many bench players can say that.
Small Sample Size Alert: Since Kobe Bryant went down with a season-ending Achilles injury (precisely three games ago), Pau Gasol has averaged 16.7 points, 15.7 rebounds and 7.7 assists.
Is the "first-option" Gasol we're seeing now the same one who costarred with Bryant to lead the Los Angeles Lakers to their 16th championship in 2010? Or is it the old, reserved guy we watched hobble up the court and stand guard on the perimeter for no logical reason all season long.
If the Lakers get the former version, there's no doubt they can upset the San Antonio Spurs in the first round; that player is a franchise pillar.
When Danilo Gallinari tore his ACL, the Denver Nuggets saw their title chances take a major hit. They're deep, but withstanding the loss of a 6'10" starting small forward seemed impossible.
Then, rookie Frenchman Evan Fournier had a few big games in an increased role (17 points against the Houston Rockets, 24 points against the Portland Trail Blazers, 12 points against the Milwaukee Bucks) and a lot of people figured the Nuggets just might be OK.
If Fournier can spread the floor, assume some pick-and-roll responsibility and defend his position, the Nuggets could possibly reach their potential, which is the Western Conference Finals.
Andrew Bogut has only appeared in 32 games this season, and during 28 of them, he played fewer than 30 minutes. (He went 1-of-8 in 17 minutes in the season finale and offered nine minutes of uninspired basketball on April 11.)
When healthy, he's a huge force near the basket, especially on the defensive end.
But his health is a major question mark, and if he can't contribute longer than 30 minutes a night as expected, the Golden State Warriors will find themselves severely shorthanded against a Denver Nuggets team that loves to attack the paint.