Source : Webster's Revised Unabridged Dictionary (1913)

Voluntary \Vol"un*ta*ry\, n.; pl. {Voluntaries}.
   1. One who engages in any affair of his own free will; a
      volunteer. [R.] --Shak.

   2. (Mus.) A piece played by a musician, often extemporarily,
      according to his fancy; specifically, an organ solo played
      before, during, or after divine service.

   3. (Eccl.) One who advocates voluntaryism.