The term acid rain, as commonly used, refers to rain, snow, or fog containing a dilute solution of sulfuric acid and nitric acid, formed mainly from pollution associated with fossil fuel combustion. Other terms for this phenomenon are acid precipitation and wet deposition. Wet deposition accounts for about half of all acidic deposition.

Acid precipitation is defined as any precipitation with a pH value of less than 5.6. Pure distilled water, a neutral substance, has a pH of 7. A pH value of less than 7 indicates an acidic material. Normal, unpolluted rain is slightly acidic, with a pH of 5.6 For more about pH, see The pH Scale.

Acid deposition also occurs through dry deposition of acidic gases and particles. This material is deposited onto trees, buildings, cars, and homes. Dry-deposited gases and particles can be washed from surfaces by rainstorms. The acidic runoff combines with wet-deposited acid precipitation and further impacts the environment.

Acid deposition readily crosses domestic and international boundaries. The tall chimney stacks used by industry to reduce local effects of pollution promote long-distance transport of the sulfur oxides and nitrogen oxides that cause acid deposition. Air pollutants remain suspended in the atmosphere for up to five days, allowing them to travel great distances before being deposited. Thus, a coal-burning power plant in the Midwest can cause acid deposition along the East Coast of the U.S.

Eastern North America and northern Europe are most known for their acid deposition problems. These forested areas of the globe have humid atmospheres, providing the water content needed to convent sulfur oxides and nitrous oxides to acids. In more arid regions such as the American West, airborne dust originating from alkaline soils provide a natural means of neutralizing sulfur oxides and nitrogen oxides. Acid deposition tends to be minimal in relatively dry areas.

The term acid rain was coined in 1852 by English chemist Robert Angus Smith, who noted a connection between London's air pollution, caused by industrial activity, and its acidic rainfall. Large-scale effects of acid deposition were not recognized until the mid-20th century. Environmental regulations to control industrial emissions of sulfur dioxide, a major source of acid deposition, began in 1990 in the United States.