自动 (zìdòng) describes a machine or process that operates by itself, without human control — 'automatic'. It contrasts with 自觉 (zìjué), which is about a person being self-aware or acting on their own initiative, and with 自行 (zìxíng), which emphasizes doing something independently or voluntarily. 自动 is for mechanical/technical automation; 自觉 is for human awareness/discipline.