facultative fac·ul·ta·tive (fāk'əl-tā'tĭv)
Capable of functioning under varying environmental conditions. Used of certain organisms, such as bacteria that can live with or without oxygen.
Capable of occurring along various pathways or under various conditions.
Capable of existing under varying environmental conditions or by assuming various behaviors. Bacteria that are facultative aerobes can live in both aerobic and anaerobic environments. A facultative parasite can live independently of its usual host. Compare obligate.