General:
* Voluntary: This is the most common and general word for something done willingly.
* Optional: This implies that there is a choice, and something is not obligatory.
* Elective: This suggests a deliberate and conscious choice, often in a formal setting.
More specific:
* Autonomous: Emphasizes self-governing and independent choice.
* Uncoerced: Highlights the absence of pressure or force in the decision.
* Spontaneous: Implies a natural and unforced choice.
* Self-determined: Focuses on the individual's agency and control over the choice.
Other options:
* By choice
* Of one's own volition
The best word to use will depend on the specific context and the meaning you want to convey.