n. The actual exertion of power; power exerted; strength in action; vigorous operation.n. Activity considered as a characteristic; habitual putting forth of power or strength, physical or mental, or readiness to exert it.n. The exertion of or capacity for a particular kind of force; action or the power of acting in any manner; special ability or agency: used of the active faculties or modes of action regarded severally, and often in the plural: as, creative energy; the energies of mind and body.n. In the Aristotelian philos., actuality; realization; existence; the being no longer in germ or in posse, but in life or in esse: opposed to power, potency, or potentiality.n. A fact of acting or actually being.n. In rhetoric, the quality of awakening the imagination of the reader or hearer, and bringing the meaning of what is said home to him; liveliness.n. In physics: Half the sum of the masses of the particles of a system each multiplied by the square of its velocity; half the vis viva. See vis viva.n. Half the greatest value to which the sum of the masses of all the particles of a given system each multiplied by the square of its velocity, could attain except for friction, viscosity, and other forces dependent on the velocities of the particles; otherwise, the amount of work (see work) which a given system could perform were it not for resistance dependent on the velocities.n. of water in motion, or in an elevated position;n. of air in motion, as the wind;n. the muscular energy of animals. To these might be added the energy of direct solar radiation, the energy of the tides, and some others of less importance. The source of all these forms of energy, except that of the tides, is to be found in the radiant energy of the sun.