tautness vs tonicity

tautness and tonicity both are nouns.

Word	Noun	Adjective	Verb	Adverb
tautness	Yes	No	No	No
tonicity	Yes	No	No	No

As nouns, tonicity is a hyponym of tautness; that is, tonicity is a word with a more specific, narrower meaning than tautness:

tautness: the physical condition of being stretched or strained
tonicity: the elastic tension of living muscles, arteries, etc. that facilitate response to stimuli

Other hyponyms of tautness include tone, tonus.

tautness (noun)	tonicity (noun)
lack of movement or room for movement	the elastic tension of living muscles, arteries, etc. that facilitate response to stimuli
the physical condition of being stretched or strained