ones' complement

Short definition from Wiktionary

Noun

1. 1
   The number obtained by subtracting a given n-digit binary number from 2ⁿ-1 (which yields the same result as the logical complement).
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 0 to -(2ⁿ⁻¹-1), n being the word size of the machine, and the numeric complement of a number is its ones' complement.