Supplying a deficiency.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (adjective) (adj.) (adjective)
Forming, or of the nature of, a complement, or completing part; complementary angles, two angles which together make a right angle, or 90 degrees.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (adjective) (adj.) (adjective)
Filling up; completing; supplying a deficiency.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (adjective) (adj.) (adjective)

Usage examples:

But in those spheres of social activity, dealing especially with certain relations between human creatures because of their diverse if complementary relation to the production of human life, the sexes as sexes have often each a part to play which the other cannot play for them; have each a knowledge gained from phases of human experience, which the other cannot supply; here woman as woman has something radically distinct to contribute to the sum- total of human knowledge, and her activity is of importance, not merely individually, but collectively, and as a class.
- Woman and Labour by Olive Schreiner
I had permitted him to hold a limited number of shares and to act nominally as secretary in order to comply with the regulations of the Security and Exchange Commission, but now it was expedient to add to our officers directors of other companies whose fields were complementary to ours.
- Greener Than You Think by Ward Moore
His studies of reflections and of complementary colours speak for this.
- The French Impressionists (1860-1900) by Camille Mauclair