Flex-Fuel - A varied mixture of fuels, typically gasoline and ethanol. Vehicles with multiple fuel systems - such as might run on hydrogen cells and gasoline, for instance - are called bi-fuel or dual fuel vehicles. To be technical, all automobiles are flex fuel vehicles if they can take a mix of gas and ethanol without modification, and most cars on the road today can. However, a true flex fuel vehicle can go from one hundred percent gasoline to one hundred percent ethanol and back.

Fluorocarbon - Nonflammable, heat-stable hydrocarbon liquid or gas, in which some or all hydrogen atoms have been replaced by fluorine atoms. As with CFC's, fluorocarbons, traditionally used as propellants (spray cans), are classified as ozone-depleting substances.

Fly Ash - Fine, noncombustible particulate primarily resulting from the combustion of coal in furnaces and kilns. Often used as a filler material in concrete to displace virgin raw materials.

Fossil Fuels - (Mainly coal, oil, and natural gas) are decayed fossils. The combustion of fossil fuels releases carbon dioxide into the atmosphere, contributing to the greenhouse effect and climate change.

Fuel cell – A technology that uses an electrochemical process to convert energy into electrical power. Often powered by natural gas, fuel cell power is cleaner than grid-connected power sources. In addition, hot water is produced as a by-product.

Fuel Efficiency - A form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is often illustrated as a continuous energy profile. Non-transportation applications, such as industry, benefit from increased fuel efficiency, especially fossil fuel power plants or industries dealing with combustion, such as ammonia production during the Haber process.

Fungi - Typically filamentous, eukaryotic, nonchlorophyllic microorganisms. Fungi grow on dead or dying organic matter and may also grow on some building materials where excess moisture is present. Fungi can cause pungent odors, unsightly stains, and premature biodeterioration of interior furnishings.