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[noun kom-pluh-muh nt; verb kom-pluh-ment] /noun ˈkɒm plə mənt; verb ˈkɒm pləˌmɛnt/
something that completes or makes perfect:
A good wine is a complement to a good meal.
the quantity or amount that completes anything:
We now have a full complement of packers.
either of two parts or things needed to complete the whole; counterpart.
full quantity or amount; complete allowance.
the full number of officers and crew required on a ship.
  1. a word or group of words that completes a grammatical construction in the predicate and that describes or is identified with the subject or object, as small in The house is small or president in They elected her president.
  2. any word or group of words used to complete a grammatical construction, especially in the predicate, including adverbials, as on the table in He put it on the table, infinitives, as to go in They are ready to go, and sometimes objects, as ball in He caught the ball.
Geometry. the quantity by which an angle or an arc falls short of 90° or a quarter of a circle.
Compare supplement (def 4).
Also called absolute complement. Mathematics. the set of all the elements of a universal set not included in a given set.
Music. the interval that completes an octave when added to a given interval.
  1. a system in vertebrate blood of 12 or more proteins that react in a cascade to a cell displaying immune complexes or foreign surfaces, acting in various combinations to coat the cell and promote phagocytosis, make holes in the cell wall, or enhance the inflammatory response.
  2. any of the proteins in the complement system, designated C1, C2, etc.
verb (used with object)
to complete; form a complement to:
This belt complements the dress better than that one.
Obsolete. to compliment.
verb (used without object)
Obsolete. to compliment.
Origin of complement
1350-1400; Middle English < Latin complēmentum something that completes, equivalent to complē(re) to fill up (see complete) + -mentum -ment
Related forms
complementer, noun
Can be confused
complement, supplement (see synonym study at the current entry)
complement, compliment (see usage note at the current entry)
Synonym Study
12. Complement, supplement both mean to make additions to something. To complement is to provide something felt to be lacking or needed; it is often applied to putting together two things, each of which supplies what is lacking in the other, to make a complete whole: Two statements from different points of view may complement each other. To supplement is merely to add to: Some additional remarks may supplement his address.
Usage note
Complement and compliment, which are pronounced alike and originally shared some meanings, have become separate words with entirely different meanings. As a noun, complement means “something that completes or makes perfect”: The rare old brandy was a perfect complement to the delicious meal. As a verb, complement means “to complete”: A bright scarf complements a dark suit. The noun compliment means “an expression of praise, commendation, or admiration”: The members paid her the compliment of a standing ovation. The verb compliment means “to pay a compliment to”: Everyone complimented him after the recital. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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British Dictionary definitions for complementer


noun (ˈkɒmplɪmənt)
a person or thing that completes something
one of two parts that make up a whole or complete each other
a complete amount, number, etc (often in the phrase full complement)
the officers and crew needed to man a ship
  1. a noun phrase that follows a copula or similar verb, as for example an idiot in the sentence He is an idiot
  2. a clause that serves as the subject or direct object of a verb or the direct object of a preposition, as for example that he would be early in the sentence I hoped that he would be early
(maths) the angle that when added to a specified angle produces a right angle
(logic, maths) the class of all things, or of all members of a given universe of discourse, that are not members of a given set
(music) the inverted form of an interval that, when added to the interval, completes the octave: the sixth is the complement of the third
(immunol) a group of proteins in the blood serum that, when activated by antibodies, causes destruction of alien cells, such as bacteria
verb (ˈkɒmplɪˌmɛnt)
(transitive) to add to, make complete, or form a complement to
Word Origin
C14: from Latin complēmentum, from complēre to fill up, from com- (intensive) + plēre to fill
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for complementer



late 14c., "that which completes," from Old French compliement "accomplishment, fulfillment" (14c., Modern French complément), from Latin complementum "that which fills up or completes," from complere "fill up" (see complete (adj.)). Originally also having senses which were taken up c.1650-1725 by compliment.



1610s, "exchange courtesies," from complement (n.). Meaning "make complete" is from 1640s. Related: Complemented; complementing.

Online Etymology Dictionary, © 2010 Douglas Harper
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complementer in Medicine

complement com·ple·ment (kŏm'plə-mənt)
A group of proteins found in normal blood serum and plasma that are activated sequentially in a cascadelike mechanism that allows them to combine with antibodies and destroy pathogenic bacteria and other foreign cells.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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complementer in Science
  1. A group of proteins in blood serum that interact systematically as part of the body's immune response to destroy disease-causing antigens, especially bacteria. Complement proteins interact with antibodies and other chemical substances to cause the disintegration of foreign cells and enhance other immune functions such as phagocytosis.

  2. A complementary color.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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