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From Citizendium, the Citizens' Compendium

Energy is a word with several connotations. The word goes back to the natural philosophy of Aristotle, ενέργεια (energeia), where it means roughly "efficacy". In the early nineteenth century the word was incorporated into science by Thomas Young. In science, the concept energy has a clear meaning, which, however, is not easy to explain because of the many forms in which energy manifests itself. The word energy is also very commonly used outside science, where it means physical or mental power to achieve something. "Negative energy" is a mental power that is in the way of achieving things.

Specifically, Energy might refer to:

• In science [r]: A measurable physical quantity of a system which can be expressed in joules (the metric unit for a quantity of energy) or other measurement units such as ergs, calories, watt-hours or Btu. [e]
• In politics [r]: Resources such as fossil fuel, biomass, solar or wind power. [e]
• In mathematics [r]: Add brief definition or description