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 fac·ul·ta·tive /ˈfækəlˌtetɪv, ||tətɪv/

 fac·ul·ta·tive /ˈfækəlˌtetɪv/ 形容詞
 兼性的

 Fac·ul·ta·tive a.
 1. Having relation to the grant 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; -- opposed to obligatory and compulsory, and sometimes used with to.
 2.  Of such a character as to admit of existing under various forms or conditions, or of happening or not happening, or the like; specif.: Biol. Having the power to live under different conditions; as, a facultative parasite, a plant which is normally saprophytic, but which may exist wholly or in part as a parasite; -- opposed to obligate.
 3.  Physiol. Pertaining to a faculty or faculties.
    In short, there is no facultative plurality in the mind; it is a single organ of true judgment for all purposes, cognitive or practical.    --J. Martineau.
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 facultative
      adj 1: of or relating to the mental faculties
      2: able to exist under more than one set of conditions; "a
         facultative parasite can exist as a parasite or a
         saprophyte" [ant: obligate]
      3: granting a privilege or permission or power to do or not do
         something; "a facultative enactment"
      4: not compulsory; "facultative courses in the sciences"