facultative
adjective
fac·ul·ta·tive ˈfa-kəl-ˌtā-tiv 
British -tə-tiv
1
a
: of or relating to the grant of permission, authority, or privilege
facultative legislation
b
: optional
2
: of or relating to a mental faculty
3
a
: taking place under some conditions but not under others
facultative diapause
b
: exhibiting an indicated lifestyle under some environmental conditions but not under others
facultative anaerobes
facultatively adverb
Example Sentences
Recent Examples on the Web As facultative carnivores, dogs draw on protein for fuel more than humans do. 
Outside Online, 5 July 2022 Because cleaner fish have other food sources besides the parasites, such as crustaceans, this relationship is also facultative mutualism. Liz Langley, National Geographic, 25 Sep. 2020 Ball pythons are native to central and western Africa and can reproduce asexually, which is known as facultative parthenogenesis, the zoo said. David Williams, CNN, 11 Sep. 2020 The dragon’s birth, which came during a study of fertility, was said to result from a special type of parthenogenesis, called facultative parthenogenesis. Martin Weil, Washington Post, 6 July 2019
Outside Online, 5 July 2022 Because cleaner fish have other food sources besides the parasites, such as crustaceans, this relationship is also facultative mutualism. Liz Langley, National Geographic, 25 Sep. 2020 Ball pythons are native to central and western Africa and can reproduce asexually, which is known as facultative parthenogenesis, the zoo said. David Williams, CNN, 11 Sep. 2020 The dragon’s birth, which came during a study of fertility, was said to result from a special type of parthenogenesis, called facultative parthenogenesis. Martin Weil, Washington Post, 6 July 2019 Word History
Etymology
borrowed from French & New Latin; French facultatif, borrowed from New Latin facultātīvus, from Medieval Latin facultāt-, facultās "ability, power, authority, privilege, right" (Latin, "ability, power") + Latin -īvus -ive — more at faculty
First Known Use
1820, in the meaning defined at sense 1a
Medical Definition
facultative
adjective
fac·ul·ta·tive ˈfak-əl-ˌtāt-iv
1
: taking place under some conditions but not under others
facultative parasitism
2
: exhibiting an indicated lifestyle under some environmental conditions but not under others
facultative anaerobes