'原因' (yuán yīn) literally combines '原' (yuán), meaning 'original' or 'source', and '因' (yīn), meaning 'cause' or 'factor'. Together, they form a compound noun meaning 'reason' or 'cause'—the underlying factor that explains why something happened or exists. It is neutral in register and widely used in both spoken and written Chinese, especially when analyzing events, problems, or outcomes.

The word often appears after question words like '什么' (shénme, 'what') or '为什么' (wèi shénme, 'why'), or with verbs like '找' (zhǎo, 'to find'), '知道' (zhīdào, 'to know'), or '分析' (fēnxī, 'to analyze'). It can refer to concrete causes (e.g., mechanical failure) or abstract reasons (e.g., emotional motivation). Unlike the more casual '理由' (lǐyóu), '原因' emphasizes objective causality rather than subjective justification.