A multiplier is a number by which another number is multiplied. What do you call a number by which another number is added or subtracted?
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There is the multiplier (that which multiplies) and the multiplicand (that which is to be multiplied).
For subtraction there is the subtrahend (that which is to be subtracted) and the minuend (that which is to be diminished).
For division there is the divisor (that which divides) and the dividend (that which is to be divided).
These words with -and or -end are Latin future participles.
There are a couple of mathematical points to make which inform the English involved.
(1) 'Addition' (yes, simple addition!) covers two major operations:
(a) Combination of two elements from a set (a binary operation) (eg 3 marbles + 2 marbles = 5 marbles).
(b) Transformation of one element into another (a unary operation) (eg a 3cm-long worm grows by 2cm )
Both are modelled identically by 3 + 2 = 5, but a transformation arrow with '+2' over the top is fitting for the transformation.
In the first case, addend + addend = sum / total
In the second case, augend + addend = sum / total.
(2) We've had a thread discussing the fact that there is no agreed term for the result of a subtraction; 'directed difference' is used by some.