Facultative

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
 * Facultative biotroph, 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 food. Compare this with the term omnivore
 * Facultative heterochromatin, tightly packed but non-repetitive DNA in the form of Heterochromatin, but which can lose its condensed structure and become transcriptionally active
 * Facultative lagoon, a type of stabilization pond used in biological treatment of industrial and domestic wastewater
 * Facultative parasite, a parasite that can complete its life cycle without depending on a host
 * Facultative photoperiodic plant, a plant that will eventually flower regardless of night length but is more likely to flower under appropriate light conditions.
 * Facultative saprophyte, lives on dying, rather than dead, plant material
 * facultative virus