The first of several addends, or "the one to which the others are added," is sometimes called the augend. Therefore, while 
, 
, and 
are addends in 
, 
is the augend.
Augend
See also
Addend, AdditionExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Augend." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Augend.html