x factor

Short definition from Wiktionary

Noun

1. 1
   Alternative spelling of X factor.
2. 2
   Alternative spelling of X factor.
3. 3
   An unknown or hard-to-define influence; a factor with unknown or unforeseeable consequences.
4. 4
   A special talent, ability or quality or its indefinable cause.
5. 5
   Prothrombinase.
6. 6
   The proportionality constant which converts CO emission line brightness to molecular hydrogen mass.
7. 7
   Alternative spelling of X factor.