Contamination is the presence of a constituent, impurity, or some other undesirable element that renders something unsuitable, unfit or harmful for both physical body, natural environment, workplace, etc.

In chemistry and materials science, impurities are chemical substances inside a confined amount of liquid, gas, or solid. They differ from the chemical composition of the material or compound. Firstly, a pure chemical should appear in at least one chemical phase and can also be characterized by its phase diagram. Secondly, a pure chemical should prove to be homogeneous. The perfect pure chemical will pass all attempts to separate and purify it further. Thirdly, and here we focus on the common chemical definition, it should not contain any trace of any other kind of chemical species. In reality, there are no absolutely 100% pure chemical compounds, as there is always some small amount of contamination.