conferring a faculty, privilege, permission, or the power of doing or not doing something: a facultative enactment.
left to one's option or choice; optional: The last questions in the examination were facultative.
that may or may not take place; that may or may not assume a specified character.
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).