Definition of X factor noun from the Oxford Advanced Learner's Dictionary

 

X factor

 noun
noun
BrE BrE//ˈeks fæktə(r)//
 
; NAmE NAmE//ˈeks fæktər//
 
[singular]
 
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a special quality, especially one that is essential for success and is difficult to describe She certainly has the X factor that all great singers have. The effect of the weather on production costs is always something of an X factor, or unknown variable.
See the Oxford Advanced American Dictionary entry: X factor