\Fac"ul*ta*tive\, a. [L. facultas, -atis, faculty:
cf. F. facultatif, G. fakultativ.]
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.