hi-fi vs high-fidelity

hi-fi is a noun but high-fidelity is not a noun.

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

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

hi-fi and high-fidelity: characterized by minimal distortion in sound reproduction

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

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