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”).