An additive is an extra substance that's added to something in order to improve it in some way. Many additives are used to preserve or flavor food.
The noun additive means "something that is added," from the Latin addere, "add to or attach." Things like gasoline, medicine, and livestock feed all contain various chemical additives that make them safer or more efficient. In math, additive is often used as an adjective, meaning "having to do with addition."