(1):

(n.) The quality or state of being spontaneous, or acting from native feeling, proneness, or temperament, without constraint or external force.

(2):

(n.) The tendency to undergo change, characteristic of both animal and vegetable organisms, and not restrained or cheked by the environment.

(3):

(n.) The tendency to activity of muscular tissue, including the voluntary muscles, when in a state of healthful vigor and refreshment.