What does facultative mean?
Facultative is a formal adjective meaning optional or discretionary — something left to individual choice rather than required. It derives from Latin facultas, meaning capability or opportunity, which also gives us faculty and facility. In everyday and legal usage, calling attendance or action facultative signals that no one is obliged to do it. In biology the word has a specialized sense: a facultative organism, such as a facultative anaerobe, can thrive under different environmental conditions, adopting one mode of life only when circumstances demand it — in contrast to an obligate organism, which cannot deviate. In insurance, facultative reinsurance refers to cover negotiated for each individual risk rather than arranged through a standing treaty. Though rarely heard in casual conversation, facultative is precise and valuable wherever the distinction between choice and necessity matters.
Done by choice or left to one's discretion; not compulsory.
"Contributions are facultative, so members may give as much or as little as they like."
(Biology) Capable of existing or functioning under more than one set of environmental conditions, adopting a particular mode of life only when necessary.
"A facultative parasite can live independently but will parasitize a host when available."
Usually opposed to obligate in this sense.
The same word that lawyers use for 'optional' lets biologists describe bacteria that can live with or without oxygen — a single term covering both free will and metabolic flexibility.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of facultative
Facultative comes from Latin facultativus, derived from facultas, meaning power, capability, or opportunity, itself rooted in facilis ('easy'), the same root behind facility, faculty, and facilitate. It entered English in the late sixteenth or early seventeenth century, initially carrying the sense of something dependent on or arising from a person's faculty or will, and soon settling into its core meaning of 'optional'. Its scientific sense developed later as biologists needed a term for organisms whose traits or behaviors are conditional rather than fixed.
Related word forms
How facultative is actually used
Formal and largely confined to academic, legal, biological, and technical registers; rare in everyday speech. The sense 'optional' is the oldest and still common in legal contexts, while the biological sense dominates modern scientific usage.
Easily confused with facultative
Facile means overly simple or achieved with little effort, whereas facultative means optional or capable of occurring only under certain conditions.