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].

Source : WordNet®

complementary angles
     n : two angles whose sum is a right angle