The Collaborative International Dictionary of English v.0.48:

Complement \Com"ple*ment\, n. [L. complementun: cf. F.
   compl['e]ment. See Complete, v. t., and cf. Compliment.]
   1. That which fills up or completes; the quantity or number
      required to fill a thing or make it complete.
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   2. That which is required to supply a deficiency, or to
      complete a symmetrical whole.
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            History is the complement of poetry.  --Sir J.
                                                  Stephen.
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   3. Full quantity, number, or amount; a complete set;
      completeness.
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            To exceed his complement and number appointed him
            which was one hundred and twenty persons. --Hakluyt.
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   4. (Math.) A second quantity added to a given quantity to
      make it equal to a third given quantity.
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   5. Something added for ornamentation; an accessory. [Obs.]
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            Without vain art or curious complements. --Spenser.
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   6. (Naut.) The whole working force of a vessel.
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   7. (Mus.) The interval wanting to complete the octave; -- the
      fourth is the complement of the fifth, the sixth of the
      third.
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   8. A compliment. [Obs.] --Shak.
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   Arithmetical compliment of a logarithm. See under
      Logarithm.

   Arithmetical complement of a number (Math.), the difference
      between that number and the next higher power of 10; as, 4
      is the complement of 6, and 16 of 84.

   Complement of an arc or Complement of an angle (Geom.),
      the difference between that arc or angle and 90[deg].

   Complement of a parallelogram. (Math.) See Gnomon.

   In her complement (Her.), said of the moon when represented
      as full.
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The Collaborative International Dictionary of English v.0.48:

Complement \Com"ple*ment\, v. t.
   1. To supply a lack; to supplement. [R.]
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   2. To compliment. [Obs.] --Jer. Taylor.
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WordNet (r) 3.0 (2006):

complement
    n 1: a word or phrase used to complete a grammatical
         construction
    2: a complete number or quantity; "a full complement"
    3: number needed to make up a whole force; "a full complement of
       workers" [syn: complement, full complement]
    4: something added to complete or embellish or make perfect; "a
       fine wine is a perfect complement to the dinner"; "wild rice
       was served as an accompaniment to the main dish" [syn:
       complement, accompaniment]
    5: one of a series of enzymes in the blood serum that are part
       of the immune response
    6: either of two parts that mutually complete each other
    v 1: make complete or perfect; supply what is wanting or form
         the complement to; "I need some pepper to complement the
         sweet touch in the soup"

The Free On-line Dictionary of Computing (19 January 2023):

complement

    The other value or values in the set of possible
   values.

   See logical complement, bitwise complement, set
   complement.

   (1995-01-24)