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com·ple·ment (kŏmplə-mənt)
a. Something that completes, makes up a whole, or brings to perfection: a sauce that is a fine complement to fish.
b. The quantity or number needed to make up a whole: shelves with a full complement of books.
c. The full crew of personnel required to run a ship.
d. Either of two parts that complete the whole or mutually complete each other.
2. An angle related to another so that the sum of their measures is 90°.
3. 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.
4. Music An interval that completes an octave when added to a given interval.
5. Immunology A complex system of proteins found in blood plasma that are sequentially activated and play various roles in the immune response, including lysing bacterial cell membranes, making pathogens more susceptible to phagocytes, and recruiting inflammatory cells to sites of infection or injury. Also called alexin.
6. Mathematics & Logic For a universal set, the set of all elements in the set that are not in a specified subset.
7. A complementary color.
tr.v. (-mĕnt) com·ple·ment·ed, com·ple·ment·ing, com·ple·ments
To serve as a complement to: Roses in a silver bowl complement the handsome cherry table.

[Middle English, from Old French, from Latin complēmentum, from complēre, to fill out; see COMPLETE.]

Usage Note: Complement and compliment, though quite distinct in meaning, are sometimes confused because they are pronounced the same. As a noun, complement means "something that completes or brings to perfection" (The antique silver was a complement to the beautifully set table); used as a verb it means "to serve as a complement to." The noun compliment means "an expression or act of courtesy or praise" (They gave us a compliment on our beautifully set table), while the verb means "to pay a compliment to."

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2022 by HarperCollins Publishers. All rights reserved.

Indo-European & Semitic Roots Appendices

    Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:

    Indo-European Roots

    Semitic Roots

    The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.