1. To corrupt, debase, or make impure by adding a foreign or inferior substance; often by replacing valuable ingredients with inferior ones; mixed with impurities.
2. To adulterate, to debase, to doctor. These verbs mean to make impure or inferior by adding foreign substances to something: to adulterate coffee with ground acorns; to have silver debased with copper; having doctored the wine with water; to use rag paper loaded with wood fiber.
3. Debased by the admixture of a foreign substance; adulterated; spurious.
4. Tainted with adultery.
5. To commit adultery.
6. To corrupt, debase, or make impure by an admixture of a foreign or a baser substance; as, to adulterate food, drink, drugs, coin, etc.
7. To defile by adultery.

Etymologically, to make impure by admixture; to corrupt from Latin adulteratus, past participle of adulterare, "to falsify, corrupt; to corrupt a woman; to commit adultery", dissimilated from ad- alterare, literally "to change, alter", from ad- and alterare, "to change, alter".