facultative

adjective

1. 1

   not compulsory

   facultative courses in the sciences

2. 2

   granting a privilege or permission or power to do or not do something

   a facultative enactment

3. 3

   able to exist under more than one set of conditions

   a facultative parasite can exist as a parasite or a saprophyte

4. 4

   of or relating to the mental faculties