Definitions of complementary:

• noun:   either one of two chromatic colors that when mixed together give white (in the case of lights) or gray (in the case of pigments)
• adjective:   of or relating to or suggestive of complementation
• adjective:   of words or propositions so related that each is the negation of the other

  Example: "`male' and `female' are complementary terms"

• adjective:   acting as or providing a complement (something that completes the whole)