volunteer >> ˌvɒlənˈtɪə
Definition: [noun] a person who freely offers to take part in an enterprise or undertake a task.
Example: a call for volunteers to act as foster-parents
Definition: [noun] a person who works for an organization without being paid.
Example: the railway is operated solely by volunteers
Definition: [verb] freely offer to do something.
Example: 140 employees volunteered for redundancy
Definition: [verb] work for an organization without being paid.
Example: volunteering is an easy way to get involved in practical conservation