Overview
Contaminant loosely refers to an undesirable or unexpected presence of some substance. Depending on the environmental or legal context, the definition may be more narrow.
From the United States Environmental Protection Agency (EPA):
The Safe Drinking Water Act (SDWA) defines “contaminant” as any physical, chemical, biological or radiological substance or matter in water. Drinking water may reasonably be expected to contain at least small amounts of some contaminants. Some contaminants may be harmful if consumed at certain levels in drinking water. The presence of contaminants does not necessarily indicate that the water poses a health risk. 1
Note that this definition explicitly indicates that contaminant has a technical meaning that differs from the connotation of a toxic or harmful pollutant. The EPA maintains Candidate Contaminant Lists that are reviewed and updated every few years, covering each kind of the contaminants mentioned. 2