Facultatively refers to the ability of an organism or system to adapt to changing conditions. Synonyms for this word include adaptively, contingently, optionally, situationally, and conditionally. These words highlight the idea that a system or organism has some degree of control over their response to changing conditions. Other synonyms for facultatively include selectively, discretely, and opportunistically. Each of these synonyms conveys the idea that the system or organism is able to make choices based on the circumstances they find themselves in. In biology, facultative refers to organisms that can live in multiple environments, so these synonyms reflect the same idea.