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

[French énergie, from Late Latin energīa, from Greek energeia, from energos, active : en-, in, at; see en-² + ergon, work.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.