Facultative

[fak-uh l-tey-tiv] /ˈfæk əlˌteɪ tɪv/

adjective
1.
conferring a faculty, privilege, permission, or the power of doing or not doing something:
a facultative enactment.
2.
left to one’s option or choice; optional:
The last questions in the examination were facultative.
3.
that may or may not take place; that may or may not assume a specified character.
4.
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 ).
5.
of or relating to the faculties.
/ˈfækəltətɪv/
adjective
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) able to exist under more than one set of environmental conditions: a facultative parasite can exist as a parasite or a saprotroph Compare obligate (sense 4)
5.
of or relating to a faculty

facultative fac·ul·ta·tive (fāk’əl-tā’tĭv)
adj.

fac’ul·ta’tive·ly adv.
facultative
(fāk’əl-tā’tĭv)
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.