voluntary in WordNet English dictionary

1. (military) a person who freely enlists for service
   volunteer  military volunteer  voluntary 
   
2. composition (often improvised) for a solo instrument (especially solo organ) and not a regular part of a religious service or musical performance
   voluntary 
   

1. of your own free will or design; done by choice; 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"
   voluntary 
   
2. controlled by individual volition
   "voluntary motions"; "voluntary muscles"
   voluntary