What is Alternative Fuel?

Legal Definition
Alternative fuels, known as non-conventional and advanced fuels , are any materials or substances that can be used as fuels, other than conventional fuels like; fossil fuels (petroleum (oil), coal, and natural gas), as well as nuclear materials such as uranium and thorium, as well as artificial radioisotope fuels that are made in nuclear reactors.

Some well-known alternative fuels include biodiesel, bioalcohol (methanol, ethanol, butanol), refuse-derived fuel, chemically stored electricity (batteries and fuel cells), hydrogen, non-fossil methane, non-fossil natural gas, vegetable oil, propane and other biomass sources.
-- Wikipedia
Legal Definition
Eco friendly substitutes for fossil fuel products.