Definition of: complement

(kom′plə·mənt) noun
1. That which fills up or completes; that which must be added to complete a symmetrical whole.
2. Full number: The vessel has her complement of men.
3. An addition or appendage; an accessory.
4. Geom. The amount by which an angle falls short of 90 degrees.
5. Gram. A word or phrase used after a verb of incomplete predication to complete the meaning of the sentence. A subjective complement describes or identifies the subject, as the noun president in He is president or in He was elected president or as the adjective happy in She is happy or in She was made happy. An objective complement describes or identifies the direct object, as the noun president in They elected him president or the adjective happy in It made her happy.
6. Music A musical interval which, with the interval already given, will complete an octave.
7. Immunology A complex system of unstable, heat-sensitive proteins normally present in human and other serums where it reacts with specific antibodies to destroy related antigens. Also called alexin.
—v.t. To add or form a complement to; supplement; make complete; supply a lack in. ◆ Homophone: compliment. [<L complementum <complere COMPLETE]
—com·ple·men·tal (kom′plə·men′təl) adjective
—com′ple·ment′er noun