Facultative means "optional" or "discretionary" (antonym obligate), used mainly in biology in phrases such as: * Facultative (FAC), facultative wetland (FACW), or facultative upland (FACU): wetland indicator statuses for plants * Facultative anaerobe, an organism that can use oxygen but also has anaerobic methods of energy production. It can survive in either environment * , an organism, often a fungus, that can live as a saprotroph but also form mutualisms with other organisms at different times of its life cycle. * Facultative biped, an animal that is capable of walking or running on two legs as well as walking or running on four limbs or more, as appropriate * Facultative carnivore, a carnivore that does not depend solely on animal flesh for food but also can subsist on non-animal