教师 (jiàoshī) is a formal term for 'teacher,' used in official titles, job descriptions, or when referring to the profession (e.g., 教师节 Teacher's Day). It is more formal than the everyday 老师 (lǎoshī), which is used as a title and in direct address (e.g., 李老师). 先生 (xiānsheng) can also mean 'teacher' in older/formal contexts but primarily means 'Mr.' or 'husband'.