Complementary is the adjective form of the word "complement." It means forming a whole or integrating to perfection or completion. The word can also mean a bonus or something pleasingly extra. For example: The bottle of beer was complementary to the delicious buffet.
The word complementary has its presence in the world of genetics. It means pertaining to a group of genes that act in conjunction with other to create a particular phenotype (a noticeable trait in an organism in terms of its size, shape, behavior and color that is the consequence of its total genetic makeup with relation to the environment).
Complementary color refers to the colors which when mixed together produce white color in case of lights, or grey color in case of pigments. A precise pairing of organic compounds pyrimidines and purines between the strands of RNA molecule or a DNA molecule in the world of bio-chemistry is also called as complementary.