Relating to or involving the right or power to act or decide; permitting choice or discretion rather than requiring it.
From Latin facultativus, from facultas 'power, ability.' Related to faculty, this term emphasizes the optional or discretionary nature of something rather than its mandatory character.
In biology, a 'facultative anaerobe' is an organism that can choose to use oxygen or not—it's got options, unlike an obligate anaerobe that absolutely must avoid oxygen!
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