Something that completes, makes up a whole, or brings to perfection. The quantity or number needed to make up a whole: shelves with a full complement of books. Either of two parts that complete the whole or mutually complete each other. An angle related to another so that the sum of their measures is 90\u00b0. Grammar A word or words used to complete a predicate construction, especially the object or indirect object of a verb; for example, the phrase to eat ice cream in We like to eat ice cream. Music An interval that completes an octave when added to a given interval. The full crew of officers and enlisted personnel required to run a ship. Immunology A complex system of proteins found in normal blood plasma that combines with antibodies to destroy pathogenic bacteria and other foreign cells. Also called alexin. Mathematics & Logic For a universal set, the set of all elements in the set that are not in a specified subset. A complementary color. To serve as a complement to: Roses in a silver bowl complement the handsome cherry table.