Acid rain (suān yǔ) is rainwater that has become unusually acidic due to atmospheric pollution—especially from sulfur dioxide and nitrogen oxides released by burning fossil fuels. The word combines 酸 (suān), meaning 'sour' or 'acidic', and 雨 (yǔ), meaning 'rain'. Together, they literally mean 'sour rain' or 'acid rain', directly reflecting its chemical nature.

This term is commonly used in environmental science, public health discussions, and ecological studies in Chinese. It appears frequently in news reports about air quality, industrial regulation, and ecosystem damage—especially when describing harm to forests, lakes, crops, and buildings. While technically a scientific term, it’s widely understood by educated speakers and often appears in school textbooks and government environmental campaigns.