Definitions of facultative:

• adjective:   granting a privilege or permission or power to do or not do something

  Example: "A facultative enactment"

• adjective:   able to exist under more than one set of conditions

  Example: "A facultative parasite can exist as a parasite or a saprophyte"

• adjective:   of or relating to the mental faculties
• adjective:   not compulsory

  Example: "Facultative courses in the sciences"