1. something that completes or makes perfect: A good wine is a complement to a good meal.
  2. the quantity or amount that completes anything: We now have a full complement of packers.
  3. either of two parts or things needed to complete the whole; counterpart.
  4. full quantity or amount; complete allowance.
  5. the full number of officers and crew required on a ship.
  6. Grammar.
    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.Compare object complement, subject complement.
    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.
  7. Geometry. the quantity by which an angle or an arc falls short of 90° or a quarter of a circle.Compare supplement(def 4).
  8. Also called absolute complement. Mathematics. the set of all the elements of a universal set not included in a given set.
  9. Music. the interval that completes an octave when added to a given interval.
  10. Immunology.
    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.
  11. complementary color.

verb (used with object)

  1. to complete; form a complement to: This belt complements the dress better than that one.
  2. Obsolete. to compliment.

verb (used without object)

  1. Obsolete. to compliment.

noun (ˈkɒmplɪmənt)

  1. a person or thing that completes something
  2. one of two parts that make up a whole or complete each other
  3. a complete amount, number, etc (often in the phrase full complement)
  4. the officers and crew needed to man a ship
  5. grammar
    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
  6. maths the angle that when added to a specified angle produces a right angle
  7. 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
  8. music the inverted form of an interval that, when added to the interval, completes the octavethe sixth is the complement of the third
  9. immunol a group of proteins in the blood serum that, when activated by antibodies, causes destruction of alien cells, such as bacteria

verb (ˈkɒmplɪˌmɛnt)

  1. (tr) to add to, make complete, or form a complement to

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.


  1. 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.

  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.
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