'备用' (bèi yòng) is an adjective meaning 'spare' or 'backup'—something kept in reserve for use when the primary item is unavailable, broken, or insufficient. The first character 备 (bèi) means 'to prepare' or 'to have ready,' while 用 (yòng) means 'to use.' Together, they literally convey 'prepared for use,' emphasizing readiness rather than immediate function. It commonly modifies nouns directly (e.g., 备用电池 'spare battery') and often appears in practical, everyday contexts like travel, technology, emergencies, or household management.

This term carries a neutral, functional tone and is widely used in both spoken and written Chinese. Unlike verbs such as '准备' (to prepare), 备用 itself is not used predicatively without a noun—it’s almost always attributive. It implies foresight and practicality, suggesting that the speaker has anticipated possible need. You’ll frequently see it on packaging, in instructions, or in conversations about contingency planning.