facultative
adj 1: of or relating to the mental faculties
2: (biology) 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"