a person who does something, especially helping other people, willingly and without being forced or paid to do it

The health clinic is relying on volunteers to run the office and answer the phones. 这家健康门诊依靠志愿者来负责办公室的日常工作和接听电话。

Since it would be a highly dangerous mission, the Lieutenant asked for volunteers. 由于那项任务非常危险，所以中尉请大家自告奋勇。

It's a volunteer army with no paid professionals. 这是一支志愿军，没有领军饷的职业军人。

Compare: conscript verb draftee

to offer to do something that you do not have to do, often without having been asked to do it and/or without expecting payment

[ + to infinitive ] During the emergency many staff volunteered to work through the weekend. 紧急时刻，很多员工都自愿周末加班。

He volunteered for the army (= he joined even though he did not have to). 他自愿参军。

I volunteered myself for the position of Health and Safety Representative. 我自告奋勇担任卫生与安全代表一职。

humorous My friends volunteered me to do the talking. 我的朋友们推举我去讲话。

to give information without being asked

If I were you, I wouldn't volunteer any details of what happened. 要是我的话，我不会主动说出所发生事情的任何细节。

[ + speech ] "I saw her going out of the main entrance half an hour ago," he volunteered. “我见她在大约半个小时之前从主出入口走了出来。”他主动说道。