two's complement

Short definition from Wiktionary

Noun

1. 1
   The number obtained by complementing every bit of a given number and adding one. A number and its complement add to 2ⁿ, where n is the word size of the machine.
2. 2
   The convention by which bit patterns with high bit 0 represent positive numbers from 0 to 2ⁿ⁻¹-1 directly, while bit patterns with high bit 1 represent negative numbers from −1 to -2ⁿ⁻¹, n being the word size of the machine, and the numeric complement of a number is its two's complement.