The term acid deposition refers to forms of precipitation (such as acid rain, acid fog or acid mist) that has elevated levels of hydrogen ions, low pH, and is therefore acidic in nature.

Acid deposition can have harmful effects on plants, animals, the environment, infrastructure and even indirectly affect human health. The principal cause of acid deposition is sulfur and nitrogen compounds released into the atmosphere by human activities, although certain natural phenomena can contribute as well.