'Yǐ wéi' is a verb meaning 'to think' or 'to believe', often implying a subjective assumption—sometimes one that turns out to be incorrect. The character 以 (yǐ) originally means 'to use' or 'by means of', and 为 (wéi) means 'to be' or 'to act as'; together, they form an expression suggesting 'taking something to be [the case]'. It functions similarly to English 'assume' or 'suppose', carrying a subtle nuance of personal judgment rather than objective fact.

This word commonly appears in clauses introduced by 'yǐ wéi + [subject] + [predicate]', especially when reporting thoughts or mistaken impressions. It's neutral in register—used in both spoken and written Chinese—and frequently occurs in narratives, conversations, and explanations where someone’s internal belief differs from reality. Unlike the simpler verb 想 (xiǎng), 'yǐ wéi' emphasizes the conclusion drawn, not just the mental process.