士兵 (shì bīng) is a formal noun for a soldier, specifically an enlisted member of the military (not an officer). It is more specific than 兵 (bīng), which can mean 'soldier' in a general sense or refer to troops collectively, and narrower than 军人 (jūn rén), which covers all military personnel including officers. 战士 (zhàn shì) is more literary/heroic, often used for 'warrior' or 'fighter'.