"facultative" in the adjective sense
of or relating to the mental faculties
able to exist under more than one set of conditions
"a facultative parasite can exist as a parasite or a saprophyte"
granting a privilege or permission or power to do or not do something
"a facultative enactment"
4. elective, facultative
"an elective course of study"
Source: WordNet® (An amazing lexical database of English)
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