'由于' (yóu yú) is a formal preposition and conjunction meaning 'due to' or 'because of'. It introduces the cause or reason for a situation, event, or result — always placed before the cause, followed by the effect in the main clause. Though composed of two characters (由 'yóu', meaning 'from' or 'by', and 于 'yú', a classical particle indicating origin or basis), together they function as a fixed compound with no separable meaning. It is common in written Chinese, academic writing, news reports, and formal speech, but rarely used in casual conversation.

The structure is typically '由于 + [reason], + [result clause]'. The reason phrase is usually a noun phrase or short clause (e.g., '天气恶劣', '资金不足'), not a full verb phrase with subject–predicate structure. Unlike the more colloquial '因为', '由于' cannot stand alone as a sentence-initial conjunction introducing an independent reason clause; it must be embedded in a larger sentence where the consequence is clearly stated.