twenty-twenty vs visual acuity

twenty-twenty and visual acuity both are nouns.

Word	Noun	Adjective	Verb	Adverb
twenty-twenty	Yes	No	No	No
visual acuity	Yes	No	No	No

As nouns, visual acuity is a hypernym of twenty-twenty; that is, visual acuity is a word with a broader meaning than twenty-twenty:

twenty-twenty: normal visual acuity, as measured by the ability to read charts at a distance of 20 feet
visual acuity: sharpness of vision; the visual ability to resolve fine detail (usually measured by a Snellen chart)

Other hypernyms of twenty-twenty include acuity, sharp-sightedness.

twenty-twenty (noun)	visual acuity (noun)
normal visual acuity, as measured by the ability to read charts at a distance of 20 feet	sharpness of vision; the visual ability to resolve fine detail (usually measured by a Snellen chart)