The term pollution is derived from the Latin pollutus, which means to be made foul, unclean, or dirty. Pollutants, then, are factors that corrupt, degrade, or make something less valuable or desirable. In environmental terms, we consider pollutants to be chemical, physical, or social factors that have an undesirable effect on a particular environment. Air pollutants, for instance, include dust, smoke, haze, foul odors, noise, and volatile (airborne) chemicals such as sulfur dioxide, carbon monoxide, lead, nitrogen oxides, acids, ambient ozone, and a large number of toxic organic compounds. Among the major water pollutants are human and animal wastes; infectious agents; oxygen-demanding organic chemicals; plant nutrients such as phosphates and nitrogen; heavy metals such as lead and mercury, acids, salts, sediment; and excess heat. Other kinds of pollutants might include visual pollution (ugly billboards), exotic biological species, cultural pollution (fashions, practices, or trends that corrupt an existing culture), or even linguistic pollution (new words from a foreign language that take over an existing language).

What might be considered pollution by one might be regarded as a welcome change by another. A toxic waste for one organism might be a highly desirable resource for another species. Oftentimes, the definition of pollution is limited to anthropogenic (human-caused) environmental changes. This can be problematic, however, because most of the materials we consider major air and water pollutants, such as dust, sediment, carbon monoxide, organic acids, and infectious agents have both human and natural sources.