X factor

noun4

1. •
   An unknown or hard-to-define influence; a factor with unknown or unforeseeable consequences.

   • We slid into our office, past two women discussing the "X factor," an apparently undefined quality their firm felt they lacked. — 1992
2. •
   A special talent, ability or quality or its indefinable cause.
3. •
   Prothrombinase.
4. •
   The proportionality constant which converts CO emission line brightness to molecular hydrogen mass.