The word 反应 is formed by combining two meaningful components: 反, which carries the core idea of "turning back" or "returning", and 应, which conveys the idea of "answering" or "responding". Their conceptual fusion creates a compound whose meaning emerges from the metaphorical image of one action or stimulus "turning back" as a reciprocal "answer", perfectly capturing the notion of a reaction, response, or reflection as seen in contexts from chemistry to human behavior.