prodigy vs brainiac

prodigy and brainiac both are nouns.

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

As nouns, brainiac is a hypernym of prodigy; that is, brainiac is a word with a broader meaning than prodigy:

prodigy: an unusually gifted or intelligent (young) person; someone whose talents excite wonder and admiration
brainiac: someone who has exceptional intellectual ability and originality

Other hypernyms of prodigy include brain, Einstein, genius, mastermind.

prodigy (noun)	brainiac (noun)
an impressive or wonderful example of a particular quality	someone who has exceptional intellectual ability and originality
a sign of something about to happen
an unusually gifted or intelligent (young) person; someone whose talents excite wonder and admiration