What is "facultative"

Facultative \Fac"ul*ta*tive\, a. [L. facultas, -atis, faculty: cf. F. facultatif, G. fakultativ.]

1. Having relation to the grant or exercise faculty, or authority, privilege, license, or the like hence, optional; as, facultative enactments, or those which convey a faculty, or permission; the facultative referendum of Switzerland is one that is optional with the people and is necessary only when demanded by petition; facultative studies; -- opposed to obligatory and compulsory, and sometimes used with to.

2. Of such a character as to admit of existing under various forms or conditions, or of happening or not happening, or the like; specif.: (Biol.) Having the power to live under different conditions; as, a facultative parasite, a plant which is normally saprophytic, but which may exist wholly or in part as a parasite; -- opposed to obligate.

3. (Physiol.) Pertaining to a faculty or faculties.

   In short, there is no facultative plurality in the mind; it is a single organ of true judgment for all purposes, cognitive or practical.
   --J. Martineau.

a. 1 Of or relating to faculty, especially to mental faculty 2 Not obligate; optional, discretionary or elective 3 That grants permission or power to do something 4 (context biology English) Able to perform a particular life function, or to live generally, in more than one way 5 (context geometry of a point English) At which a given function is positive.

1. adj. of or relating to the mental faculties

2. able to exist under more than one set of conditions; "a facultative parasite can exist as a parasite or a saprophyte" [ant: obligate]

3. granting a privilege or permission or power to do or not do something; "a facultative enactment"

4. not compulsory; "facultative courses in the sciences"

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