Originally, the term was used for a piece of organ music that was free in style, and was intended to sound improvised (the word voluntary in general means "proceeding from the will or from one's own choice or consent").
In the 18th century, many organists and other musicians of high prominence, particularly in London, published collections of voluntaries - usually ten or twelve -'for organ or harpsichord', establishing the genre as a serious addition to the standard repertoire.
Some imitate the character of the French overture, with a slow movement of dotted or double-dotted rhythms, followed by a fugue.
In the latter the eccho manual and/or the swell or full organ to give ripieno effects are often employed, and it is typically non-fugal.
1735-1784) made more use of the slow introduction than, say, John Stanley whose introductory movements tend to be shorter and simpler in comparison.
Several of the movements are named after organ solo stops or mixtures (bassoon, cornet, trumpet, sesquialtera, flute, twelfth, etc.).
In general, the English organ voluntary of the 18th century arguably does not have the more profound, devotional flavour of the chorale prelude of, say, J S Bach who had died in 1750.
Of the many composers active in London, those of Maurice Greene, William Goodwin and John Bennett arguably show the most inventiveness and variety in character.