义工(yì gōng) - voluntary work; a volunteer in Mandarin

义工 (yìgōng) is voluntary work, or a volunteer. 义 (yì) means "righteous, duty, justice" and 工 (gōng) means "work, labour", so together the word carries the sense of work done out of duty or principle rather than for pay.

It covers both the activity and the person: 做义工 (zuò yìgōng), "to do voluntary work", and 当义工 (dāng yìgōng), "to be a volunteer", are both common, and 义工 itself can stand for the volunteer, as in "她是义工" (tā shì yìgōng), "she is a volunteer". A near-synonym, 志愿者 (zhìyuànzhě), leans slightly more formal and is what you would see on an official sign-up form; 义工 is the everyday word.

For English speakers, the trap is reaching for a literal translation of "volunteer work" built from 志愿 (zhìyuàn), "to volunteer", plus 工作 (gōngzuò, from gong1-zuo4), "job". That phrase is understandable but clunky; 义工 is the word a native speaker actually says, whether the context is 学校 (xuéxiào), school, a hospital, or a local 社会 (shèhuì, from she4-hui4) charity project.