In the world of mathematics, an "x-factor" is an unknown quantity which only becomes known after following a prescribed process.
This concept of an unknown but vital quality also extends into other worlds, such as business and entertainment. Interviewers and judges often speak of an x-factor as a certain undefinable quality which may promote one candidate over another in the eyes of his or her critics or examiners.
The x-factor for an actor or actress might be a strong sexual appeal or a sense of mystery, for instance.
When it comes to playoff basketball in the NBA, an x-factor is a player who serves as a catlyst, equipped with skills and talent that directly result in team success.
We saw several players step up to get their teams out of the first round.
Now, who is going to rise even further in the second round matchups and be this year's ultimate x-factor?