facultative

/ˈfækəlˌteɪtɪv/
adjective
  1. Optional or not required; able to be chosen or not chosen.
    • The course includes a facultative field trip that students can skip if they prefer.
    • Attendance at the workshop is facultative, so only those interested need to come.
    • The contract has a facultative clause allowing either party to withdraw without penalty.
  2. In biology, able to live or function in more than one way, especially under different environmental conditions.
    • Some plants are facultative perennials, meaning they can act as annuals in harsh climates.
    • The bacteria are facultative parasites, able to live independently or inside a host.
    • Facultative anaerobes can survive with or without oxygen.