1. The elastic tension of living muscles, arteries, etc. that facilitate response to stimuli.
2. The normal elastic tension of living muscles, arteries, etc., by which the tone of the system is maintained.
3. A state of normal tension of the tissues by virtue of which the parts are kept in shape, alert, and ready to function in response to a suitable stimulus.

In the case of muscle, it refers to a state of continuous activity or tension beyond that which is related to physical properties; i.e., it is active resistance to stretch; in skeletal muscle it is dependent upon the efferent innervation.

4. The osmotic pressure or tension of a solution, usually relative to that of blood.