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Wiktionary
additive inverse

n. (context mathematics English) The inverse with respect to addition; the opposite.

WordNet
additive inverse

n. (mathematics) one of a pair of numbers whose sum is zero; the additive inverse of -5 is +5

Wikipedia
Additive inverse

In mathematics, the additive inverse of a number is the number that, when added to , yields zero. This number is also known as the opposite (number), sign change, and negation. For a real number, it reverses its sign: the opposite to a positive number is negative, and the opposite to a negative number is positive. Zero is the additive inverse of itself.

The additive inverse of is denoted by unary minus: − (see the discussion below). For example, the additive inverse of 7 is −7, because 7 + (−7) = 0, and the additive inverse of −0.3 is 0.3, because −0.3 + 0.3 = 0 .

The additive inverse is defined as its inverse element under the binary operation of addition (see the discussion below), which allows a broad generalization to mathematical objects other than numbers. As for any inverse operation, double additive inverse has no net effect: .