(1):
(a.) Pertaining to a faculty or faculties.
(2):
(a.) 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):
(a.) Of such a character as to admit of existing under various forms or conditions, or of happening or not happening, or the like;
(4):
(a.) 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.