An Acid Deposition is a complex chemical and atmospheric phenomenon that occurs when Emissions of sulfur and nitrogen compounds and other substances are transformed by

chemical processes in the atmosphere, often far from the original sources, and then deposited on earth in either wet or dry form. The wet forms, popularly called "acid rain," can fall to earth as rain, snow, or fog. The dry forms are acidic gases or particulates. (EPA 1992, 1)

Other definition:

An acid deposition is the input of acid-forming pollutants to a body of water. This situation has caused significant environmental damage in the Northeastern United States. Sulfur and nitrogen pollutants are responsible for most of the acidification.

Other definition:
Acid deposition is the falling of acids and acid-forming compounds from the atmosphere to the earth"s surface. Acid deposition is commonly known as Acid rain, a term that refers to the wet deposition of droplets of acids and acid-forming compounds.