tonicity vs tonus

tonicity and tonus both are nouns.

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

As nouns, tonicity and tonus are synonyms defined as:

tonicity and tonus: the elastic tension of living muscles, arteries, etc. that facilitate response to stimuli

Other synonyms of tonicity include tone.

tonicity (noun)	tonus (noun)
the elastic tension of living muscles, arteries, etc. that facilitate response to stimuli	the elastic tension of living muscles, arteries, etc. that facilitate response to stimuli