additive inverse; opposite number
In mathematics, the number that when added to a given number yields zero. For example, the additive inverse of 5 is -5.
The additive inverse of 5 is -5.
reciprocal number
Rare mathematical term for the reciprocal (逆数). Cross-referenced with 逆数, which is the standard modern term. This usage is uncommon and may be considered outdated or nonstandard.
See also: 逆数 (ぎゃくすう)
In old mathematics books, 反数 is sometimes used to mean the reciprocal.