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The Collaborative International Dictionary of English v.0.48 (gcide)

Facultative \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.
[Webster 1913 Suppl.]

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}.
[Webster 1913 Suppl.]

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.
[Webster 1913 Suppl.]