Definition of energy

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en·er·gy    (nr-j) KEY 

NOUN:
pl. en·er·gies

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.

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ETYMOLOGY:
French énergie, from Late Latin energa, from Greek energeia, from energos, active : en-, in, at ; see en-² + ergon, work; see werg- in Indo-European roots