(1) The muscle in a steady partially contracted state caused by the successive flow of nerve impulses.
(2) The amount of tension or resistance to movement in a muscle.
Muscle tone or tonus helps in maintaining body posture. For instance, the muscles holding the jaw keep it in position by contracting continuously and only partially. When a person loses consciousness or sleeps, the muscle tone declines.
Word origin: tonus » Greek tónos (tone)