- The numerical result obtained when a number is subtracted from the radix, which is the number of digits used in a numbering system. For instance, the complement of 6 in the decimal number system is 4: (10 - 6) = 4. The complement of a number in the binary number system is the other: 1 is the complement of 0, and 0 is the complement of 1. Used in computers, for instance, to represent negative numbers. Also called complement, or true complement.