tonus vs tonicity

tonus and tonicity both are nouns.

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

As nouns, tonus and tonicity are synonyms defined as:

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

Other synonyms of tonus include tone.

tonus (noun)	tonicity (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