fiend vs demon

fiend and demon both are nouns.

Word	Noun	Adjective	Verb	Adverb
fiend	Yes	No	No	No
demon	Yes	No	No	No

As nouns, fiend and demon are synonyms defined as:

Other synonyms of fiend include devil, monster, ogre.

As nouns, fiend and demon are synonyms defined as:

Other synonyms of fiend include daemon, daimon, devil.

fiend (noun)	demon (noun)
a cruel wicked and inhuman person	an evil supernatural being
a person motivated by irrational enthusiasm (as for a cause)	a cruel wicked and inhuman person
an evil supernatural being	someone extremely diligent or skillful