Source : Webster's Revised Unabridged Dictionary (1913)

Complementary \Com`ple*men"ta*ry\, a.
   Serving to fill out or to complete; as, complementary
   numbers.

   {Complementary colors}. See under {Color}.

   {Complementary angles} (Math.), two angles whose sum is
      90[deg].

Complementary \Com`ple*men"ta*ry\, n. [See {Complimentary}.]
   One skilled in compliments. [Obs.] --B. Jonson.

Source : WordNet®

complementary
     n : either one of two chromatic colors that when mixed together
         give white (in the case of lights) or gray (in the case
         of pigments); "yellow and blue are complementaries" [syn:
          {complementary color}]

complementary
     adj 1: acting as or providing a complement (something that
            completes the whole) [syn: {complemental}, {completing}]
     2: of words or propositions so related that each is the
        negation of the other; "`male' and `female' are
        complementary terms"
     3: of or relating to or suggestive of complementation;
        "interchangeable electric outlets" [syn: {interchangeable},
         {reciprocal}]