Pollution was the insertion of toxic or otherwise contaminant materials into an environment. It could take the form of chemicals, or energy such as radiation, noise, heat or light. An ecosystem could be damaged or destroyed when exposed to pollution.
When visiting 20th century Earth in a time travel accident, Quark could not believe that humans would detonate nuclear fission bombs within their own atmosphere, polluting their planet. (DS9 episode: "Little Green Men")