Source : Webster's Revised Unabridged Dictionary (1913)

Tonus \To"nus\, n. [L. a sound, tone. See {Tone}.] (Physiol.)
   Tonicity, or tone; as, muscular tonus.

Source : WordNet®

tonus
     n : the elastic tension of living muscles, arteries, etc. that
         facilitate response to stimuli; "the doctor tested my
         tonicity" [syn: {tonicity}, {tone}] [ant: {atonicity}]