Volunteer: a person who freely offers to take part in an enterprise or undertake a task. $ c9 E1 m1 n1 v/ e2 Nsynonyms: subject, participant, case, patient; informalguinea pig ( [1 c) ~6 @' y8 B, O Z6 w"each volunteer was tested three times" ! i. O8 K D" I. h/ ?