It describes a compound or chemical that has been determined to be harmful to living organisms at certain known doses. A contaminant is a substance or compound that has the potential to become toxic. Contaminants are often thought of as chemicals or compounds not found naturally in an ecosystem and which have the potential to cause harm to organisms or populations of organisms. In general, the words toxic and contaminant are used interchangeably, though as noted above, the word toxic is more properly an adjective, and the word contaminant a noun. The terms chemical and contaminant are often confused.
Some chemicals are very toxic, of course, while others are very beneficial.
Most medicines, for example, could be described as chemicals. As with toxic or contaminant the term assumes that we already know that the compound is harmful. The main point here is that no one wants to put contaminants into the Chesapeake Bay.