(astronomy) An angle which, together with a given angle, makes a right angle. [from 18th c.]
Something which completes, something which combines with something else to make up a complete whole; loosely, something perceived to be a harmonious or desirable partner or addition. [from 19th c.]
(grammar) A word or group of words that completes a grammatical construction in the predicate and that describes or is identified with the subject or object. [from 19th c.]
(music) An interval which, together with the given interval, makes an octave. [from 19th c.]
(optics) The color which, when mixed with the given color, gives black (for mixing pigments) or white (for mixing light). [from 19th c.]
(set theory) Given two sets, the set containing one set's elements that are not members of the other set (whether a relative complement or an absolute complement). [from 20th c.]
(logic) An expression related to some other expression such that it is true under the same conditions that make other false, and vice versa. [from 20th c.]
(electronics) A voltage level with the opposite logical sense to the given one.
(computing) A bit with the opposite value to the given one; the logical complement of a number.
(genetics) A nucleotide sequence in which each base is replaced by the complementary base of the given sequence: adenine (A) by thymine (T) or uracil (U), cytosine (C) by guanine (G), and vice versa.