What do we call rain that has become acidic from pollution?

Acid rain. Sulphur dioxide and nitrogen oxides from burning coal and oil react with water in the atmosphere to make sulphuric and nitric acid, which then falls, often hundreds of kilometres downwind of whatever produced it. Rain is naturally slightly acidic anyway from dissolved carbon dioxide; the polluted version can be a hundred times more so.

The damage was severe and international. Lakes across Scandinavia and eastern Canada lost their fish, forests in central Europe died back, and limestone buildings and statues dissolved visibly - much of the erosion on old cathedrals happened in a single century rather than over the preceding several. The politics were poisonous because the emissions and the damage were in different countries.

It is also one of the clearest environmental success stories anyone has, which tends to get forgotten. Cap-and-trade rules on sulphur in the United States from 1990, and equivalent European agreements, cut emissions faster and far more cheaply than industry predicted, and the affected lakes have been recovering since. The problem did not solve itself and has not entirely gone away, but it is the standard proof that this kind of regulation can work.