酸雨为酸性沉降中的湿沈降，酸性沉降可分为「湿沈降」与「干沈降」两大类，前者指的是所有气状污染物或粒状污染物，随着雨、雪、雾或雹等降水形态而落到地面者，后者则是指在不下雨的日子，从空中降下来的落尘所带的酸性物质而言。酸雨又分硝酸型酸雨和硫酸型酸雨。

在化学上定义水之酸碱值等于七为中性，小於则是酸性。自然大气中含有大量二氧化碳，二氧化碳在常温时溶解于雨水中并达到气液相平衡后，雨水之酸碱值约为5.6，因此大自然的雨水是酸的。

但是，在大自然中，仍存在其他致酸的物质，例如，火山爆发所喷出的硫化氢，海洋所释放出的二甲基硫，高空闪电所导致之氮氧化物等，均会使雨水进一步酸化，而酸碱值会降至5.0左右。因此，在1980年代后期以来，许多国内外（包含环保署研究报告）研究者，已将所谓「酸雨」认知为当雨水酸碱值在5.0以下时，即确定受到人为酸性污染物的影响。

Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions (low pH). It can have harmful effects on plants, aquatic animals and infrastructure. Acid rain is caused by emissions of sulfur dioxide and nitrogen oxide, which react with the water molecules in the atmosphere to produce acids. Some Governments have made efforts since the 1970s to reduce the release of sulfur dioxide and nitrogen oxide into the atmosphere with positive results. Nitrogen oxides can also be produced naturally by lightning strikes, and sulfur dioxide is produced by volcanic eruptions. The chemicals in acid rain can cause paint to peel, corrosion of steel structures such as bridges, and weathering of stone buildings and statues.