high fidelity vs hi-fi

high fidelity and hi-fi both are nouns.

Word	Noun	Adjective	Verb	Adverb
high fidelity	Yes	No	No	No
hi-fi	Yes	Yes	No	No

As nouns, high fidelity and hi-fi are synonyms defined as:

high fidelity and hi-fi: the reproduction of sound with little or no distortion

high fidelity (noun)	hi-fi (noun)
the reproduction of sound with little or no distortion	the reproduction of sound with little or no distortion
	equipment for the reproduction of sound with high fidelity

high fidelity (adjective)	hi-fi (adjective)
	characterized by minimal distortion in sound reproduction