Foreign matter refers to any substance or material that is not normally present in a particular environment. There are several synonyms for this term, including contaminants, impurities, foreign bodies, pollutants, and alien substances. These descriptors can refer to a wide range of things, from pollutants in the air or water to debris on a manufacturing assembly line. Other synonyms might include waste products or substances that have been introduced accidentally or intentionally. Regardless of the specific term used, the underlying concept is the same: something that does not belong in a certain place or context and can disrupt or contaminate that environment.