1. optional or discretionary; not required or mandatory
2. able to adapt to different conditions or environments
Usage: biology
3. granting permission or authority; enabling
Usage: formal

Examples

• The course includes both required readings and facultative assignments.
• Attendance at the workshop is facultative for all employees.
• Some bacteria are facultative anaerobes that can survive with or without oxygen.
• The plant shows facultative responses to different light conditions.
• The new policy provides facultative guidelines rather than strict rules.
• Facultative parasites can live independently or on a host organism.