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t 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; -- opp…

Anaërobic bacteria. They are called facultative anaërobia when able to live either in the presence or absence of free oxygen; obligate, or obligatory, anaërobia when they thrive only in its absence.