Any substance that makes the environment dirty or contaminated is a pollutant. Soot is an example of an air pollutant, and chemical pollutants can make their way into the water supply.
Trash that doesn't quickly biodegrade is one kind of pollutant—think of plastic bags floating around in the ocean. Chemicals that find their way into the air we breathe and the water we drink are another kind of pollutant. Pollutant comes from pollution, from the Late Latin pollutionem, "defilement," and its root polluere, "to soil or defile."