Name of the value being added 
Multiplier is to multiplication as $x$ is to addition

What is $x$ called?
I could not figure out the right search term to find this word but I know that one must exist.
If this is not the right domain to ask in please let me know!
 A: By analogy to the multiplier in the Powerball lottery game, the term you're be looking for might be "bonus."
For example, in the Pennsylvania lottery's Powerball Match 5, a Power Play multiplier is chosen prior to the drawing. Let's say that multiplier is "3X" meaning prizes are tripled. A $10,000 prize would be tripled to \$30,000. Or, more likely for me, my \$2 winning would be tripled to \$6.
A raffle at work that I participated in a few years ago had a bonus of \$100. One of the VPs threw that in: an extra \$100 for anyone who won any prize. As usual, I matched the maximum amount of numbers to still not win anything.
One of the commenters suggested increment. As the commenter said, a multiplier gets applied (multiplied by) to several different numbers, like the prize amounts in a lottery. Likewise, an increment gets applied (added to) several different numbers, like the prize amounts in a raffle.
A: https://en.wikipedia.org/wiki/Addition#Notation_and_terminology

The numbers or the objects to be added in general addition are collectively referred to as the terms,[4] the addends[5][6] or the summands;[7] this terminology carries over to the summation of multiple terms. This is to be distinguished from factors, which are multiplied. Some authors call the first addend the augend.[5][6] In fact, during the Renaissance, many authors did not consider the first addend an "addend" at all. Today, due to the commutative property of addition, "augend" is rarely used, and both terms are generally called addends.[8]
All of the above terminology derives from Latin. "Addition" and "add" are English words derived from the Latin verb addere, which is in turn a compound of ad "to" and dare "to give", from the Proto-Indo-European root *deh₃- "to give"; thus to add is to give to.[8] Using the gerundive suffix -nd results in "addend", "thing to be added".[a] Likewise from augere "to increase", one gets "augend", "thing to be increased".
"Sum" and "summand" derive from the Latin noun summa "the highest, the top" and associated verb summare. This is appropriate not only because the sum of two positive numbers is greater than either, but because it was common for the ancient Greeks and Romans to add upward, contrary to the modern practice of adding downward, so that a sum was literally higher than the addends.[10] Addere and summare date back at least to Boethius, if not to earlier Roman writers such as Vitruvius and Frontinus; Boethius also used several other terms for the addition operation. The later Middle English terms "adden" and "adding" were popularized by Chaucer.[11]

