An additive is an unnecessary ingredient added to a mixture in order to enhance it in some way. Food additives might be used as a sweetener or preservative. Additives are put into auto paint in order to make it more flexible, or to give it a pearl or metallic appearance.
Additive also means relating to addition; Zero is called the additive identity, because adding zero to a number will not change it. The additive property of an equation stats that if you add the same number to each side of an equation, the two sides remain equal.