tonicity vs tautness

tonicity and tautness both are nouns.

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

As nouns, tautness is a hypernym of tonicity; that is, tautness is a word with a broader meaning than tonicity:

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

Other hypernyms of tonicity include tenseness, tension, tensity.

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