hi-fi vs headroom

hi-fi and headroom both are nouns.

Word	Noun	Adjective	Verb	Adverb
hi-fi	Yes	Yes	No	No
headroom	Yes	No	No	No

As nouns, headroom is a hyponym of hi-fi; that is, headroom is a word with a more specific, narrower meaning than hi-fi:

hi-fi: the reproduction of sound with little or no distortion
headroom: the capacity of a system to reproduce loud sounds without distortion

Other hyponyms of hi-fi include dynamic headroom.

hi-fi (noun)	headroom (noun)
the reproduction of sound with little or no distortion	the capacity of a system to reproduce loud sounds without distortion
equipment for the reproduction of sound with high fidelity	vertical space available to allow easy passage under something

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