Massachusetts Technology Collaborative's Energy Glossary
Organized alphabetically and by topic.
www.mtpc.org/cleanenergy/energy/glossary.htm

U.S. Environmental Protection Agency's Clean Energy Glossary
Written with adults in mind.
www.epa.gov/cleanenergy/glossary.htm

U.S. Department of Energy, Energy Information Administration

Though this glossary is found on the Energy Kids Page, it uses many terms that could be challenging for kids to understand.
www.eia.doe.gov/kids/glossary/index.html