自行 (zì xíng) is a formal adverb meaning 'by oneself, on one's own', often used in written or official contexts (e.g., 自行决定 'decide on one's own'). It differs from 自己 (zì jǐ), which is a pronoun ('oneself'), and from 单独/独自 (dān dú/dú zì), which emphasize being alone rather than acting independently. 自行 focuses on the action being self-initiated, not necessarily solitary.