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From Biology-Online Dictionary
(1) Not compulsory; not restricting.
(2) Of, or relating to, mental faculty.
(biology) Capable of existing to more than one set of (environmental) conditions or by assuming various behaviors.
For example, facultative bacteria can thrive in both aerobic and anaerobic environments.
Word origin: NL facultātīvus.
Related forms: facultatively (adjective).
Compare: obligate (biology).