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.
      [1913 Webster]

   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.]
      [1913 Webster]

   2. To compliment. [Obs.] --Jer. Taylor.
      [1913 Webster]

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"

Moby Thesaurus II by Grady Ward, 1.0:

267 Moby Thesaurus words for "complement":
   A to Z, A to izzard, IC analysis, accession, accessory,
   accompaniment, add to, addenda, addendum, additament, addition,
   additive, additory, additum, adjunct, adjuvant, age group,
   aggregate, all, all and sundry, allowance, ally, alpha and omega,
   alter ego, analogon, analogue, annex, annexation, answer,
   answer to, appanage, appendage, appendant, appositive,
   appurtenance, appurtenant, assemblage, associate, attachment,
   attribute, attributive, augment, augmentation, bakehead, band,
   battalion, be-all, be-all and end-all, beginning and end, bellyful,
   bevy, black gang, body, boilerman, brigade, brother, bumper, bunch,
   bungs, cabal, cabin boy, capacity, cast, charge, chips, clique,
   close copy, close match, coda, coequal, cognate, cohort,
   commissary steward, companion, company, complete, completion,
   concomitant, confirmation, congenator, congener,
   construction modifier, consummation, contingent, continuation,
   coordinate, corollary, corps, correlate, correlative, correspond,
   correspond to, correspondent, coterie, counterpart, covey, cram,
   crew, crowd, crush, cutting, deckhand, deckie, deep structure,
   detachment, detail, direct object, division, each and every, eight,
   eleven, enhance, enhancement, enrichment, equivalent, everything,
   extension, extrapolation, faction, fellow, fill, filler, fireman,
   first string, first team, five, fixture, fleet, form-function unit,
   full house, full measure, function, gang, group, grouping,
   groupment, gun loader, gunner, hand, hospital steward, image,
   immediate constituent analysis, in-group, increase, increment,
   indirect object, jam up, junta, kindred spirit, lading,
   landing signalman, length and breadth, levels, like, likeness,
   load, mail orderly, makeweight, mate, mob, modifier, mouthful,
   movement, navigator, near duplicate, nine, object, obverse,
   offshoot, oiler, one and all, out-group, outfit, pack, package,
   package deal, parallel, party, peer group, pendant, perfect,
   perfection, phalanx, phrase structure, picture, platoon, posse,
   predicate, purser, qualifier, quorum, quota, radio operator, ranks,
   reciprocal, reciprocate, regiment, reinforcement, reserves,
   respond to, round out, roustabout, rowing crew, salon, second self,
   second string, second team, set, set off, shallow structure,
   side effect, side issue, similitude, simulacrum, sister, skinful,
   slot, slot and filler, snip, snips, snootful, soul mate, sparks,
   squad, stable, steward, stewardess, stoker, strata, string,
   structure, subject, such, suchlike, supplement, surface structure,
   syntactic analysis, syntactic structure, syntactics, syntax,
   tagmeme, tailpiece, tally, team, the corpus, the ensemble,
   the entirety, the like of, the likes of, the lot, the whole,
   the whole range, third string, top off, torpedoman, tribe, troop,
   troupe, twin, undergirding, underlying structure, varsity, watch,
   wing, word arrangement, word order, yeoman



The Free On-line Dictionary of Computing (30 December 2018):

complement

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

   See logical complement, bitwise complement, set
   complement.

   (1995-01-24)