[n] composition (often improvised) for a solo instrument (especially solo organ) and not a regular part of a service or performance
[n] a person who freely enlists for (military) service
[adj] of your own free will or design; not forced or compelled; "man is a voluntary agent"; "participation was voluntary"; "voluntary manslaughter"; "voluntary generosity in times of disaster"; "voluntary social workers"; "a voluntary confession"
[adj] (physiology) controlled by individual volition; "voluntary motions"; "voluntary muscles"