facultative (Adjective) — Of or relating to the mental faculties.
facultative (Adjective) — (biology) able to exist under more than one set of conditions. ex. "a facultative parasite can exist as a parasite or a saprophyte"
facultative (Adjective) — Granting a privilege or permission or power to do or not do something. ex. "a facultative enactment"
facultative (Adjective) — Not compulsory. ex. "facultative courses in the sciences"