The word “voluntary” can be used for the title of a piece of music. The title was often used by English composers in the late Renaissance or Baroque periods for a piece of organ music that was free in style, i.e. it did not have to be composed in a strict form such as sonata form or a fugue. It was meant to sound as if it was being improvised (the word voluntary in general means “free”, i.e. not “forced to do something”).
Composers such as Orlando Gibbons, John Blow, Sigismund von Neukomm, and Henry Purcell wrote voluntaries, although sometimes they preferred to use other titles such as fancy (an English form of the Italian word fantasia), or even fugue. However, these fugues were not composed in the proper fugue style: they just started off with imitation as in a fugue, but continued in a freer style.
Some voluntaries were called double voluntaries. These were pieces written for organs with two manuals (keyboards). The pieces contrasted a loud manual with a soft one.