conferring a faculty, privilege, permission, or the power of doing or not doing something: a facultative enactment.
left to one's option or choice; optional: The last questions in the examination were facultative.
that may or may not take place; that may or may not assume a specified character.
Biology. having the capacity to live under more than one specific set of environmental conditions, as a plant that can lead either a parasitic or a nonparasitic life or a bacterium that can live with or without air (opposed to obligate ).
of or pertaining to the faculties.

1810–20; < Neo-Latin facultātīvus. See faculty, -ive

facultatively, adverb
nonfacultative, adjective
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World English Dictionary
facultative (ˈfækəltətɪv)
1.  empowering but not compelling the doing of an act
2.  philosophy that may or may not occur
3.  insurance denoting a form of reinsurance in which the reinsurer has no obligation to accept a particular risk nor the insurer to reinsure, terms and conditions being negotiated for each reinsurance
4.  biology Compare obligate able to exist under more than one set of environmental conditions: a facultative parasite can exist as a parasite or a saprotroph
5.  of or relating to a faculty

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American Heritage
Medical Dictionary

facultative fac·ul·ta·tive (fāk'əl-tā'tĭv)

  1. Capable of functioning under varying environmental conditions. Used of certain organisms, such as bacteria that can live with or without oxygen.

  2. Capable of occurring along various pathways or under various conditions.

fac'ul·ta'tive·ly adv.
The American Heritage® Stedman's Medical Dictionary
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American Heritage
Science Dictionary
facultative   (fāk'əl-tā'tĭv)  Pronunciation Key 
Capable of existing under varying environmental conditions or by assuming various behaviors. Bacteria that are facultative aerobes can live in both aerobic and anaerobic environments. A facultative parasite can live independently of its usual host. Compare obligate.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Naturally it is not a fixation upon a given strategy, but a facultative
  flexibility in learning from others.
The survivors of the oxygen crisis would have mostly been facultative anaerobes.
Facultative mimicry: cues for colour change and colour accuracy in a coral reef
The facility uses facultative bacteria to nitrify the leachate ammonium to
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