Addend is a math term for any number that's added to another. If you add 10 to 15, then 10 is the addend. You can also call an addend a summand.
When it comes to addition, the numbers you're working with are addends—the equivalent in multiplication would be called factors. If you're adding 7 to 112, the 7 is an addend and the second number is officially called an augend, although you'll rarely hear anyone except a math teacher use either of these terms. The Latin root of addend is addere, "to add to, join, or attach."