genius vs brainiac

genius and brainiac both are nouns.

Word	Noun	Adjective	Verb	Adverb
genius	Yes	No	No	No
brainiac	Yes	No	No	No

As nouns, genius and brainiac are synonyms defined as:

genius and brainiac: someone who has exceptional intellectual ability and originality

Other synonyms of genius include brain, Einstein, mastermind.

genius (noun)	brainiac (noun)
exceptional creative ability	someone who has exceptional intellectual ability and originality
a natural talent
unusual mental ability
someone who has exceptional intellectual ability and originality
someone who is dazzlingly skilled in any field