Definition of: facultative

(fak′əl·tā′tiv) adjective
1. Producing or imparting faculty or power; enabling; qualifying.
2. Endowing with authority or power, but allowing the use of it at option or contingently.
3. Empowering but not requiring one to perform some act; providing, as a law, for optional action: distinguished from obligative.
4. Biol. Having the power to exist in and become adapted to changed conditions, as aerobic bacteria, which may become anaerobic: distinguished from obligate.
5. Related or pertaining to a faculty or faculties.