Put simply, pollution is the damage caused to any environment by harmful substances or waste. Most often pollution concerns the natural environment – water pollution, air pollution, noise pollution, soil contamination, thermal pollution, light pollution, visual pollution, radioactive contamination, plastic pollution, and littering. Other types of pollution include mental pollution, genetic pollution, space pollution, etc.
Until the 16th century, the verb 'to pollute' and the noun 'pollution' were used mainly in relation to morals and religion, thus pollution was defined as ceremonial impurity or defilement, or as profanation of some thing or place held to be sacred.
The term pollution is generally used to describe the presence of chemical substances produced by man's activities but not to describe the natural existence of even higher levels of the same substance. Thus, if a factory discharges lead into a river, even if it is diluted down to harmless levels, this is considered to pollute the river; but the lead naturally leached from rocks, even yielding high levels which can be dangerous to fish, is not considered to be a pollutant. Such loose usage of the term 'pollution' is biased and the word should be employed only when damage occurs.