geothermal: Energy within the Earth in the form of heat.

greenhouse effect: The warming of the planet caused by gases in the atmosphere trapping the sun’s heat instead of letting it get through to space. This action is very similar to what happens in a greenhouse.

greenhouse gases: Gases such as carbon dioxide, methane, and nitrous oxide produced by the burning and processing of fossil fuels and that contribute to global warming and acid rain.

greywater : Water already used for washing, laundry, or showering that is appropriate for household functions from toilet flushing to watering plants.