自动 (zìdòng) is an adjective meaning 'automatic' or 'self-acting'—describing something that operates without human intervention. The first character 自 (zì) means 'self' or 'by oneself', and the second character 动 (dòng) means 'to move' or 'to act'. Together, they literally convey 'self-movement' or 'action arising from within', which evolved into the modern sense of automation. It commonly modifies nouns like 门 (mén, door), 模式 (módì, mode), or 系统 (xìtǒng, system).

This word appears frequently in technology, transportation, and home appliances—for example, automatic doors in subway stations, automatic gear shifts in cars, or automatic lighting systems. While it can occasionally function as a noun (e.g., in technical contexts like 'the auto'), its primary and most natural usage is adjectival. It’s neutral in register and widely understood across all age groups in both spoken and written Mandarin.