volunteer
/ˌvɑlənˈtɪr/
(military) a person who freely enlists for service
Chinese meaning: 志愿者；自愿者
Many volunteers joined the search and rescue operation after the earthquake.
地震后许多志愿者加入了搜救行动。
He served as a volunteer in the army for two years.
他在军队中作为志愿者服役了两年。