An adulterant is caused by the act of adulteration, a practice of secretly mixing a substance with another. Typical substances that are adulterated include but are not limited to food, cosmetics, pharmaceuticals, fuel, or other chemicals, that compromise the safety or effectiveness of the said substance.

It will not normally be present in any specification or declared substances due to accident or negligence rather than intent, and also for the introduction of unwanted substances after the product has been made. Adulteration, therefore, implies that the adulterant was introduced deliberately in the initial manufacturing process, or sometimes that it was present in the raw materials and should have been removed, but was not.

An adulterant is distinct from, for example, permitted food preservatives. There can be a fine line between adulterant and additive; chicory may be added to coffee to reduce the cost or achieve a desired flavor—this is adulteration if not declared, but may be stated on the label. Chalk was often added to bread flour; this reduces the cost and increases whiteness, but the calcium confers health benefits, and in modern bread, a little chalk may be included as an additive for this reason.

In wartime, adulterants have been added to make foodstuffs "go further" and prevent shortages. The German word ersatz is widely recognised for such practices during World War II. Such adulteration was sometimes deliberately hidden from the population to prevent loss of morale and propaganda reasons. Some goods considered luxurious in the Soviet Bloc such as coffee were adulterated to make them affordable to the general population.