voluntary

(noun) (military) a person who freely enlists for service. Synonyms: volunteer, military volunteer.

(noun) composition (often improvised) for a solo instrument (especially solo organ) and not a regular part of a religious service or musical performance.

(adj) of your own free will or design; done by choice; not forced or compelled. Synonyms: voluntary (vs. involuntary). Examples: "Man is a voluntary agent." "Participation was voluntary." "Voluntary manslaughter." "Voluntary generosity in times of disaster." "Voluntary social workers." "A voluntary confession."

(adj) controlled by individual volition. Synonyms: voluntary (vs. involuntary). Examples: "Voluntary motions." "Voluntary muscles."