(1) 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 related to the physical properties; i.e., it is active resistance to stretch; in skeletal muscle it is dependent upon the efferent innervation. SYN: tonus. (2) The osmotic pressure or tension of a solution, usually relative to that of blood.isotonicity. [G. tonos, tone]