Other forms: X factors
An X factor is something truly special about a person or thing, a quality that's hard to put into words. Your sister's a great singer, but it's her X factor that really won the talent show.
No one but you knows the X factor that makes your famous chocolate chip cookies extra delicious (a sprinkle of flaked sea salt on top). It's that certain something which helps them stand out from ordinary cookies and makes them exceptional. The idiomatic X factor is nearly synonymous with je ne sais quoi, an impossible-to-define quality. In math, an X factor is "an unknown quantity that only becomes known after following a prescribed process."