acid rain

This term describes a specific environmental phenomenon where precipitation becomes acidic due to industrial pollutants. It is primarily used in scientific, ecological, and educational contexts to discuss environmental degradation and atmospheric chemistry.

As a mass noun, it refers to the phenomenon in general. While one might discuss "instances" or "events" of acid rain, the term itself is treated as an uncountable substance, meaning it does not have a plural form and cannot be counted with numbers.