optional (adj.)
- alternative
- arbitrary
- autonomous
- discretional
- discretionary
- disjunctive
- elective
- free
- gratuitous
- independent
- nonmandatory
- self-acting
- spontaneous
- unasked
- uncalled-for
- uncoerced
- unforced
- uninfluenced
- uninvited
- unprompted
- unrequested
- unsolicited
- unsought
- volitional
- voluntary
- volunteer
- willful
