‘反应’ (fǎn yìng) literally combines ‘反’ (fǎn), meaning ‘to reverse’ or ‘against’, and ‘应’ (yìng), meaning ‘to respond’ or ‘to answer’. Together, they form a word that describes a response—either physical, chemical, emotional, or behavioral—to a stimulus. As a noun, it refers to the response itself (e.g., an allergic reaction); as a verb, it means ‘to react’ (e.g., ‘He reacted quickly’). It’s widely used in medical, scientific, psychological, and everyday contexts.

The word is neutral in register and appears frequently in both spoken and written Chinese. It often follows verbs like ‘有’ (yǒu, ‘to have’) or ‘产生’ (chǎnshēng, ‘to produce’) when used as a noun, and takes subject–verb–object structure when used as a verb. Unlike English ‘reaction’, which can imply negativity, ‘反应’ is context-dependent—it can be positive, negative, or neutral (e.g., a favorable reaction to news).