a person who undertakes a service of their own free will, e.g.:
a person who does something without being under any legal or moral obligation to do so
a person who enters into military service voluntarily
(used before a noun) being a volunteer; consisting of or engaged in by volunteers
a volunteer army
volunteer activities to help the homeless
(used before a noun) said of a plant or crop: growing spontaneously without direct human control, cultivation, or supervision, esp from seeds dropped from a previous crop
a volunteer plant or crop
[obsolete French voluntaire, from Latin voluntarius: see voluntary1]