To adulterate something means to make it impure or weaker by adding inferior or harmful substances to it. When someone adulterates food, they might mix in cheaper ingredients to increase profits while deceiving customers about what they're really buying. For example, a dishonest merchant might adulterate honey by mixing in corn syrup, or adulterate olive oil by blending it with cheaper vegetable oil.

You might read about scientists testing products to make sure they haven't been adulterated, or food safety inspectors checking that milk hasn't been watered down and that spices are pure. The word can apply to anything that gets corrupted: someone might say that a beautiful landscape was adulterated by ugly billboards, or that a clear message was adulterated by confusing additions.

When something is described as unadulterated, it means pure and unmixed: unadulterated orange juice contains nothing but oranges, and unadulterated joy means happiness with nothing holding it back.