证书 (zhèng shū) refers to an official certificate or diploma that proves a qualification, such as a language certificate or academic degree. It is more specific than 证明 (zhèng míng 'proof/evidence') or 证 (zhèng 'ID card'), which are broader terms for any document that verifies a fact. 证件 (zhèng jiàn) is for personal identification (e.g., ID card), while 证书 focuses on earned credentials.