Either 1 in "1+1"

• Sum thing
• The 3 or 5 in 3 + 5 = 8
• Number next to a plus sign
• Sum component
• Either 2, in ''2+2''
• Total part
• Part of the total
• Part of a sum

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
addend \addend\ n. 1. a number that is added to another number. [WordNet 1.5] ||

WordNet Word definitions in WordNet
n. a number that is added to another number (the augend)