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" when they consider a number of candidates for a single open position. In this sense, an X factor is a certain undefinable quality which may promote one candidate over another in the eyes of his or her critics or examiners. The X factor is something rarely included in a job description, but candidates who possess it are more likely to survive early cuts during the culling process.

The X factor for an actor or actress might be a strong sexual appeal or a sense of mystery, for instance. The casting director may be able to gather 200 young and attractive actresses for a cattle call audition, but only a handful may possess the X factor of a Marilyn Monroe or Angelina Jolie. Hundreds of actors may audition for the role of master spy James Bond, but very few may have the X factor of mystery required for a successful portrayal. Many casting directors and producers are keenly aware of the particular X factor they seek during the auditioning process.

In the business world, the X factor could be a significant amount of personal charisma or an air of ambition. These qualities would be difficult to quantify, but employers and human resource specialists can usually separate candidates with a certain X factor from those who do not possess it. The position may require a high level of diplomacy or discretion or personal charm, qualities which are generally considered x factors during the interview process.

The X factor definitely becomes a consideration when judging competitive talent events such as American Idol. All of the contestants may demonstrate competent singing and dancing skills, and they may be able to communicate a song to the audience. Only a few singers, however, possess an X factor which would propel them into the rarefied air of super stardom. The X factor could be a strong personality or a sense of artistry while interpreting the music. When contestants in a creative contest are fairly equal in terms of training and ability, it often comes down to the X factor before a winner can be determined.