A volunteer is someone who does work without being paid for it, because they want to do it.

A volunteer is someone who offers to do a particular task or job without being forced to do it.

If you volunteer to do something, you offer to do it without being forced to do it.

If you volunteer information, you tell someone something without being asked.

A volunteer is someone who chooses to join the armed forces, especially during a war, as opposed to someone who is forced to join by law.