The capacity for work or vigorous activity; vigor; power. See Synonyms at strength. Exertion of vigor or power: a project requiring a great deal of time and energy. Vitality and intensity of expression: a speech delivered with energy and emotion. Usable heat or power: Each year Americans consume a high percentage of the world's energy. A source of usable power, such as petroleum or coal. Physics The capacity of a physical system to do work.