Facultative - Definition
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"