X factor

• x-factor
• x factor

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X factor (plural X factors)

1. (idiomatic) An unknown or hard-to-define influence; a factor with unknown or unforeseeable consequences.
   • 1992, Lia Matera, Prior Convictions, page 121:

     We slid into our office, past two women discussing the "X factor," an apparently undefined quality their firm felt they lacked.

   • 1999, Joel R. DeLuca, Political Savvy: Systematic Approaches to Leadership Behind the Scenes, page 75:

     The map can be misleading. It can appear to contain all the relevant information, but in real life, there is always an X factor.

   • 2007, Tasmina Perry, Daddy's Girls, page 12:

     And put him on an LA film set and he glowed with that indefinable X-factor that agents the world over wished they could bottle.

   • 2007 December 31, Fareed Zakaria, “The Rise of a Fierce Fragile Superpower”, in Newsweek, page 38:

     Whether it's trade, global warming, Darfur or North Korea, China has become the new x factor, without which no durable solution is possible.

2. (idiomatic) A special talent, ability or quality or its indefinable cause.
3. Prothrombinase, also called factor X.
4. (astrophysics) The proportionality constant which converts CO emission line brightness to molecular hydrogen mass.