Facultative
WordNet
adjective
(1) Not compulsory
"Facultative courses in the sciences"
(2) Granting a privilege or permission or power to do or not do something
"A facultative enactment"
(3) Able to exist under more than one set of conditions
"A facultative parasite can exist as a parasite or a saprophyte"
(4) Of or relating to the mental faculties