hi-fi vs high fidelity sound system

hi-fi and high fidelity sound system both are nouns.

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

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

hi-fi and high fidelity sound system: equipment for the reproduction of sound with high fidelity

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

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