penambahan suatu unsur tidak murni, murah, atau yang tidak berguna untuk menipu, menjadikan lebih murah, atau memalsukan suatu preparat; dalam terminologi hukum, pemberian label yang tidak benar, termasuk dosis yang tidak sesuai dengan label.

Merriam-Webster Online Dictionary
adulteration (noun)
the process of the condition of being - adulterating adulterated
an adulterated product
Adulteration (Wikipedia)

An adulterant is a pejorative term for a substance found within other substances such as food, fuels or chemicals even though it is not allowed for legal or other reasons.

It will not normally be present in any specification or declared contents of the substance, and may not be legally allowed. The addition of adulterants is called adulteration. The most common reason for adulteration is the use by manufacturers of undeclared materials that are cheaper than the correct and declared ones. The adulterants may be harmful, or reduce the potency of the product, or they may be harmless.

The term "contamination" is usually used for the inclusion of unwanted 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 additives. There can be a fine line between adulterant and additive; chicory may be added to coffee to reduce the cost or achieve a desired flavour—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 actually 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 from such practices during WW2. Such adulteration was sometimes deliberately hidden from the population to prevent loss of morale and propaganda reasons. Some goods considered luxurious in the Communist Bloc such as coffee were adulterated to make them affordable to the general population.

Adulterants added to reduce the amount of expensive product in illicit drugs are called cutting agents. Deliberate addition of toxic adulterants to food or other products for human consumption is poisoning.