Definitions of voluntary:

• noun:   composition (often improvised) for a solo instrument (especially solo organ) and not a regular part of a service or performance
• noun:   (military) a person who freely enlists for service
• adjective:   of your own free will or design; not forced or compelled

  Example: "Man is a voluntary agent"

• adjective:   controlled by individual volition

  Example: "Voluntary motions"