‘因由’ is a literary, formal noun meaning 'cause' or 'reason' — the underlying basis for an event, action, or emotional state. It combines 因 (yīn), meaning 'cause' or 'origin', and 由 (yóu), meaning 'source', 'origin', or 'through which'. Together, they reinforce the idea of a fundamental, often implicit, reason — not just a surface explanation but something more essential or deeply rooted. This word appears frequently in classical texts, formal writing, legal documents, and serious discourse, where precision and gravitas are valued.

Unlike everyday words like 原因 (yuányīn) or 理由 (lǐyóu), 因由 carries a slightly archaic or solemn tone and is rarely used in casual speech. It often implies complexity or hidden motivation — for instance, in describing someone’s unspoken grievance or a historical turning point. Learners should recognize it in reading but generally avoid using it conversationally unless aiming for deliberate literary effect.