Summand
... is the same in English and German.
In an addition problem a summand is what is being added or something to which something else is added. For instance, in 4+3 both the 4 and 3 are summands. In 1+2+3+4 there are actually four summands: the 1, 2, 3 and four. The result of an addition is called the sum.
4 + 3 + 9 + 5 = 21
4: first summand
3: second summand
9: third summand
5: fifth summand
21: the sum (of them all)