1. A mixture of methane and carbon dioxide along with traces of other gases, such as nitrogen, hydrogen, and water vapor, that is produced during anaerobic digestion.
2. A combustible gas produced by microbial activity, usually referring to methane produced by microbial fermentation of organic wastes.
3. Any gas fuel derived from the decay of organic matter, as the mixture of methane and carbon dioxide produced by the bacterial decomposition of sewage, manure, garbage, or plant crops.
4. A gaseous fuel of medium energy content, composed of methane and carbon dioxide; produced from the anaerobic decomposition of organic material in land fills.