willpower vs will power

willpower and will power both are nouns.

Word	Noun	Adjective	Verb	Adverb
willpower	Yes	No	No	No
will power	Yes	No	No	No

As nouns, willpower and will power are synonyms defined as:

willpower and will power: the trait of resolutely controlling your own behavior

Other synonyms of willpower include possession, self-command, self-control, self-possession, self-will.

willpower (noun)	will power (noun)
the trait of resolutely controlling your own behavior	the trait of resolutely controlling your own behavior