A pollutant was a substance or energy which contributed to pollution.
In 2368, the homes of Corvan gilvo in the rainforests of the planet Corvan II were threatened by industrial pollutants. (TNG: "New Ground")
In 2369, when Doctor Julian Bashir arrived at a Bajoran village he received a medical emergency call from, he scanned the air, the ground water, and the soil but was not able to find any pollutants. (DS9: "The Storyteller")