What does "facultative" mean?

Facultative: 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.

Additional senses

  1. 2.(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.

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