未婚 is a formal attributive adjective meaning 'unmarried'. It is often used in legal or official contexts, such as in 未婚妻 (fiancée) or 未婚夫 (fiancé), and in forms like 未婚状态 (marital status: unmarried). It differs from 单身, which can refer to being single in a broader sense (not in a relationship) and is less formal. Remember that 未婚 is typically placed directly before a noun, not used as a predicate (e.g., you would not say 她未婚 — instead use 她还没结婚 or 她是未婚的状态).