facultative
1. [ adjective ] granting a privilege or permission or power to do or not do something
Examples:

"a facultative enactment"

Related terms: enabling
2. [ adjective ] (biology) able to exist under more than one set of conditions
Examples:

"a facultative parasite can exist as a parasite or a saprophyte"

Related terms: obligate biology
3. [ adjective ] of or relating to the mental faculties
Related terms: faculty
4. [ adjective ] not compulsory
Examples:

"facultative courses in the sciences"

Related terms: optional
Similar spelling:   facilitative
  faecalith