facultative

adjective

1. conferring a faculty, privilege, permission, or the power of doing or not doing something: a facultative enactment.
2. left to one’s option or choice; optional: The last questions in the examination were facultative.
3. that may or may not take place; that may or may not assume a specified character.
4. Biology. having the capacity to live under more than one specific set of environmental conditions, as a plant that can lead either a parasitic or a nonparasitic life or a bacterium that can live with or without air (opposed to obligate).
5. of or relating to the faculties.

adjective

1. empowering but not compelling the doing of an act
2. philosophy that may or may not occur
3. insurance denoting a form of reinsurance in which the reinsurer has no obligation to accept a particular risk nor the insurer to reinsure, terms and conditions being negotiated for each reinsurance
4. biology able to exist under more than one set of environmental conditionsa facultative parasite can exist as a parasite or a saprotroph Compare obligate (def. 4)
5. of or relating to a faculty

adj.

1. Capable of functioning under varying environmental conditions. Used of certain organisms, such as bacteria that can live with or without oxygen.
2. Capable of occurring along various pathways or under various conditions.

1. 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.