visual acuity vs 20/20

visual acuity and 20/20 both are nouns.

Word	Noun	Adjective	Verb	Adverb
visual acuity	Yes	No	No	No
20/20	Yes	No	No	No

As nouns, 20/20 is a hyponym of visual acuity; that is, 20/20 is a word with a more specific, narrower meaning than visual acuity:

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

Other hyponyms of visual acuity include twenty-twenty, oxyopia.

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