An Etymological Dictionary of Astronomy and Astrophysics

1. Math., logic: With reference to any set A, conceived as a subset of some larger set U, all the elements of U that are not contained in A.
   

2. Optics: A complementary color that, when combined to another, creates neutral gray.

Etymology (EN): From O.Fr. complement, from L. complementum “that which fills up or completes,” from complere “fill up,” → complete.

Etymology (PE): Osporân, from ospor present tense stem of osporidan “to → complete.”