An adulterant is a chemical which acts as a contaminant when combined with other substances.
Adulterants are added to pure substances to extend the quantity while reducing the quality.
Examples of Adulterants
When water is added to alcohol, the water is an adulterant.
In the food and drug industry, many more examples of adulterants may be found. When cutting agents are added to drugs to reduce their expense, the added substances are considered to be adulterants. Melamine has been added to milk and other protein-containing foods to boost crude protein content, often at risk of sickness or death. High fructose corn syrup is added to adulterate honey. Injecting water or brine into meat increases its weight and is an adulterant. Diethylene glycol is a dangerous additive found in some sweet wines.
Adulterant vs Additive
An additive is an ingredient added to a product for a specific purpose (not to reduce quality). In some cases, it's difficult to tell an additive and adulterant apart. For example, chicory was first added to coffee to extend it (an adulterant), but now may be added to impart a special flavor (an additive). Chalk may be added to bread flour to reduce its cost (an adulterant), but it is often used as an additive for making bread because it increases calcium content and whiteness.
Usually an additive is listed as an ingredient, while an adulterant is not. There are exceptions. For example, adding water to meat to increase its weight (and thus manufacturer profit) is listed on the label, yet confers no benefit to the consumer.