IDIOMS

X factor

(astrophysics) The proportionality constant which converts CO emission line brightness to molecular hydrogen mass.

Example: 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.
prothrombinase, also called factor X.