What is the name of the number that is being added to another number? What is the name of the number that is being added to another number? 
I am writting a paper, and I want to know how to call the number that you add to another number (for example: 2+4, how can I call the four without saying the word "4"). Thanks.
 A: The word summand would denote $2$ as well as $4$ in your example, and is not what you want.
Then, augend would denote the $2$ alone, and addend the $4$ alone.
A: Addend is not a very familiar term, whereas summand makes no distinction between the number being added to and the number being added. Perhaps the term you are looking for is simply increment.  This is certainly in common use in calculus when you speak of $\Delta x$ as being the $x$-increment and evaluate $f$ at $x+\Delta x$.
A: Either addend or summand is correct.  If you are adding more than two numbers, it might be more appropriate to use the term addend or summand to refer to any term being added, e.g.
$$\text{summand} + \text{summand} + \dotsb + \text{summand} = \text{sum (or total)} $$
or
$$\text{addend} + \text{addend} + \dotsb + \text{addend} = \text{sum (or total)}. $$
If you need to refer to a specific term, the phrase "the $n$-th summand (or addend)" gets the job done.
In a case when you are adding exactly two numbers, you can use augend and addend, to refer to the first and second terms in a summation, i.e. you can write
$$\text{augend} + \text{addend} = \text{sum}.$$
That being said, this use of the term "addend" is not unambiguous: for example, Mathworld defines "summand" as a synonym of addend, with augend referring to the first summand (or addend) in any sum.  Hence it might be inadvisable to use "addend" to refer to only the second term in a sum, as the term "addend" is not unambiguously understood in this way; it could refer to any term in a sum.  Moreover, the term "augend" is somewhat esoteric, and its use will likely draw requests for clarification.
A: The word you're looking for is addend.
