Facultative means "optional" or "discretionary" (antonym obligate), used mainly in biology in phrases such as:
Facultative anaerobe, an organism that can use oxygen but also has anaerobic methods of energy production. It can survive in either environment
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
