"Addend" is the name given to any one of a set of terms to be added.
3 is an addend.
2 is an addend.
5 is the sum.
2.3x + 3.5x = 5.8x
2.3x is an addend.
3.5x is an addend.
5.8x is the sum.
(2x + 5y) + (7x - 4y) = 9x + y
(2x + 5y) is an addend.
(7x - 4y) is an addend.
9x + y is the sum.