缘故 (yuán gù) is a formal noun meaning 'reason' or 'cause', often used to explain why something happened. The character 缘 (yuán) originally refers to a 'karmic connection' or 'circumstance that brings things together', while 故 (gù) means 'event', 'incident', or 'former matter'. Together, they convey the idea of an underlying cause or justification—especially one that’s not immediately obvious or that carries some weight in explanation.

This word appears frequently in written Chinese and formal speech, such as explanations in news reports, official notices, or polite conversation. It’s rarely used alone; instead, it commonly follows phrases like 由于 (yóu yú, 'due to'), 因为…的缘故 (yīn wèi…de yuán gù, 'because of…'), or 有…的缘故 (yǒu…de yuán gù, 'for the reason of…'). While similar to 原因 (yuán yīn), 缘故 sounds slightly more literary and implies a subtle, sometimes intangible, cause—not just a simple factual reason.