Acid rain: Precipitation that has become acidic -- that is, has a low pH factor -- caused by the emission of sulfur oxides from fossil-fuel burning power plants.

Alternative fuels: A popular term for "non-conventional" fuels used for transportation and derived from natural gas, such as propane and compressed natural gas, or biomass materials such as methanol or ethanol.

Attic fan: A fan mounted on an exterior wall in an attic wall that transfers warm attic air to the outside.

Attic vent: A passive or mechanical device used to ventilate an attic space, primarily to reduce heat buildup and moisture condensation.

Biodiesel: A biofuel made by combining animal fat or vegetable oil (such as soybean oil or recycled restaurant grease) with alcohol that can be directly substituted for diesel as a stand-alone fuel -- often called B100, for 100 percent biodiesel. It also can be used as an additive -- often called B20, for 20 percent bio-diesel. Biodiesel can generally be used in vehicles manufactured after 1994.

Biofuel: Biofuels are renewable liquid fuels -- primarily ethanol and biodiesel -- made from plant matter rather than fossil fuels. Biofuels can help reduce air toxics emissions, greenhouse gas buildup and dependence on imported oil, while supporting United States agriculture.

Biomass: A type of renewable fuel that includes trees and other crops and residues, solid waste, sewage and liquid fuels derived from agricultural products. Some of the common energy sources derived from biomass are landfill gas, anaerobic digester gas, methane and biofuels including biodiesel, bio-oil and ethanol.

British thermal unit (Btu): The amount of heat required to raise the temperature of 1 pound of water 1 degree Fahrenheit. A Btu is equal to 252 calories.

Calorie: The term can be expressed two ways. Expressed as a "small calorie," it means the amount of heat required to raise the temperature of 1 gram of water 1 degree Celsius. Expressed as a "large calorie" or "kilogram-calorie," it represents the amount of heat required to raise 1 kilogram (1,000 grams) of water 1 degree Celsius. When the term is capitalized it indicates a kilogram-calorie.

Carbon dioxide: A colorless, odorless noncombustible gas present in the earth's atmosphere. It is designated with the formula CO2 and is formed by the combustion of carbon and carbon compounds (such as fossil fuels and biomass), by respiration (a slow combustion in animals and plants) or by the gradual oxidation of organic matter in the soil.

Carbon monoxide: A colorless, odorless but poisonous combustible gas with the formula CO. Carbon monoxide is produced in the incomplete combustion of carbon and carbon compounds such as fossil fuels (coal, petroleum) and their products (liquefied petroleum gas, gasoline) and biomass.