WordWeb Online

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"
    
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"

See also: enabling, faculty, optional

Antonym: obligate