1. The microbial conversion of a chemical into a compound of economic importance, or the conversion of organic material into a source of energy, using biological processes or organisms.
2. Any of various processes that use plants or micro-organisms to change one form of energy into another form; such as, the fermentation of carbohydrates into alcohol; the digestion of organic wastes, or sewage, with micro-organisms to produce methane.
3. The conversion of biomass into usable energy; such as, by burning solid fuel for heat, by fermenting plant matter to produce liquid fuel, as ethanol, or by the bacterial decomposition of organic waste to produce methane.