complement

\ˈkäm-plə-mənt\ noun

1 a: something that fills up, completes, or makes perfect b: the quantity, number, or assortment required to make a thing complete; especially : the whole force or personnel of a ship c: one of two mutually completing parts : counterpart
2 a: the angle or arc that when added to a given angle or arc equals a right angle in measure b: the set of all elements that do not belong to a given set and are contained in a particular mathematical set containing the given set c: a number that when added to another number of the same sign yields zero if the significant digit farthest to the left is discarded —used especially in assembly language programming
With thanks to m-w.com/dictionary