ADJECTIVE (4)

1. of or relating to the mental faculties;

2. able to exist under more than one set of conditions;
- Example: "a facultative parasite can exist as a parasite or a saprophyte"

3. granting a privilege or permission or power to do or not do something;
- Example: "a facultative enactment"

4. not compulsory;
- Example: "facultative courses in the sciences"

The Collaborative International Dictionary of English v.0.48:

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.]

WordNet (r) 3.0 (2006):

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"