WordWeb Online

1. Granting a privilege or permission or power to do or not do something
   "a facultative enactment"
    
2. Optional
   - elective
    
3. Of or relating to the mental faculties
    
4. (biology) able to exist under more than one set of conditions
   "a facultative parasite can exist as a parasite or a saprophyte"

See also: enabling, optional

Antonym: obligate

Encyclopedia: Facultative