Volunteer: a person who freely offers to take part in an enterprise or undertake a task./ ?6 m4 g7 R3 y6 W# @% q9 p
synonyms: subject, participant, case, patient; informalguinea pig1 d$ f8 L8 q l4 i! w* `- Z
"each volunteer was tested three times"9 Q% }! L; g: d+ r& [