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