Thus voluntary and involuntary, which are antonyms of each other, are both partial synonyms of spontaneous. We speak of spontaneous generation, spontaneous combustion, spontaneous sympathy, an involuntary start, an unbidden tear, voluntary agreement, willing submission.

J. C. Fernald, English Synonyms and Antonyms (1914)

Example sentences

optional Growing old is mandatory. Growing up is optional.
volitional These are both called "future volitional" and include the intent of the subject.

Popularity over time

Word	1850 → 2019	Trend
voluntary		falling
optional		steady
volitional		rising
uncoerced		falling

Use in printed books, per decade from 1850 to 2019.

By length

4 letters: free¹
6 letters: unpaid², wilful
7 letters: willing², willful², unasked, unwaged
8 letters: designed², intended³, optional³, honorary⁴, elective³, purposed, unforced, unbidden
9 letters: conscious², arbitrary⁴, purposive, uncoerced
10 letters: autonomous⁴, deliberate⁴, gratuitous⁴, volitional, unsalaried
11 letters: intentional⁴, uncompelled
12 letters: discretional
13 letters: discretionary⁵, unconstrained³

The small number after each word is its number of syllables (unpaid² = 2 syllables).

In other languages

Français: volontaire