This term refers to an environment lacking in molecular oxygen (O₂), or to an organism, tissue, chemical reaction, or biological process that does not require oxygen. Anaerobic organisms can use a molecule other than O₂ as the terminal electron acceptor in respiration. These organisms can be either obligate, meaning that they cannot use O₂, or facultative, meaning that they do not require oxygen but can use it if it is available.

Organic matter decomposition in poorly aerated environments, including water-logged soils, septic tanks, and anaerobically-operated waste treatment facilities, produces large amounts of methane gas. The methane can become an atmospheric pollutant, or it may be captured and used for fuel, as in "biogas"-powered electrical generators. Anaerobic decomposition produces the notorious "swamp gases" that have been reported as unidentified flying objects (UFOs).