Something that serves as a complement to something else.

The number which, added to the given n-digit number in radix r, results in rⁿ. In binary (r=2), this is the two's complement.

(set theory) The set that contains exactly those elements belonging to the universal set but not to a given set.

(set theory, of set A "in" set B) The set containing exactly those elements belonging to B but not to A.

(computing) The number, obtained by complementing every bit of a given number, which produces all ones when exclusive ored with the given number.

(computing) The number obtained by changing the numeric sign of a number. This may or may not be the twos complement, depending on the way numbers are stored.

(computing) The number obtained by subtracting a given n-digit binary number from 2ⁿ-1 (which yields the same result as the logical complement).

(computing) 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.

(mathematics) The relative pseudo-complement of a given element (of a Heyting algebra) with respect to the least element — the "zero" of that algebra.

(biology) The air that can be drawn into the lungs in addition to the tidal air, by the deepest possible inhalation.