The Collaborative International Dictionary of English v.0.48:

Reduction \Re*duc"tion\ (r[-e]*d[u^]k"sh[u^]n), n. [F.
   r['e]duction, L. reductio. See Reduce.]
   1. The act of reducing, or state of being reduced; conversion
      to a given state or condition; diminution; conquest; as,
      the reduction of a body to powder; the reduction of things
      to order; the reduction of the expenses of government; the
      reduction of a rebellious province.
      [1913 Webster]

   2. (Arith. & Alg.) The act or process of reducing. See
      Reduce, v. t., 6. and To reduce an equation, To
      reduce an expression, under Reduce, v. t.
      [1913 Webster]

   3. (Astron.)
      (a) The correction of observations for known errors of
          instruments, etc.
      (b) The preparation of the facts and measurements of
          observations in order to deduce a general result.
          [1913 Webster]

   4. The process of making a copy of something, as a figure,
      design, or draught, on a smaller scale, preserving the
      proper proportions. --Fairholt.
      [1913 Webster]

   5. (Logic) The bringing of a syllogism in one of the
      so-called imperfect modes into a mode in the first figure.
      [1913 Webster]

   6. (Chem. & Metal.) The act, process, or result of
      reducing[7]; as, the reduction of iron from its ores; the
      reduction of an aldehyde into an alcohol.
      [1913 Webster]

   7. (Med.) The operation of restoring a dislocated or
      fractured part to its former place.
      [1913 Webster]

   Reduction ascending (Arith.), the operation of changing
      numbers of a lower into others of a higher denomination,
      as cents to dollars.

   Reduction descending (Arith.), the operation of changing
      numbers of a higher into others of a lower denomination,
      as dollars to cents.
      [1913 Webster]

   Syn: Diminution; decrease; abatement; curtailment;
        subjugation; conquest; subjection.
        [1913 Webster]

WordNet (r) 3.0 (2006):

reduction
    n 1: the act of decreasing or reducing something [syn:
         decrease, diminution, reduction, step-down] [ant:
         increase, step-up]
    2: any process in which electrons are added to an atom or ion
       (as by removing oxygen or adding hydrogen); always occurs
       accompanied by oxidation of the reducing agent [syn:
       reduction, reducing]
    3: the act of reducing complexity [syn: reduction,
       simplification]

Moby Thesaurus II by Grady Ward, 1.0:

290 Moby Thesaurus words for "reduction":
   abasement, abatement, abbreviation, about-face, abridgment,
   abstract, acetification, acidification, acidulation, addition,
   agio, alchemy, alkalization, allayment, alleviation, allowance,
   analgesia, anesthesia, anesthetizing, apocope, appeasement,
   approximation, assimilation, assuagement, assumption, astriction,
   astringency, attenuation, attrition, bank discount, becoming,
   blunting, bottleneck, break, breakage, breaking, bump, bust,
   calming, carbonation, cash discount, casting down, catalysis,
   cervix, chain discount, change, change-over, charge-off,
   cheapening, chemicalization, circumscription, coarctation,
   compactedness, compaction, compression, compressure, concavity,
   concentration, concession, condensation, conquering, consolidation,
   conspectus, constriction, constringency, contraction, contracture,
   conversion, crushing, curtailment, cut, cutting, dampening,
   damping, de-escalation, deadening, debasement, debilitation,
   decline, decrease, decrement, decrescence, deduction, deflation,
   degradation, degrading, demotion, demulsion, depletion, depluming,
   depreciation, depression, derogation, descent, detraction,
   detrusion, devaluation, devitalization, differentiation, dilution,
   diminishment, diminuendo, diminution, dip, discount, disgrading,
   disparagement, displuming, dive, division, domestication,
   downgrading, drawback, ducking, dulcification, dulling, dying,
   dying off, ease, easement, easing, effemination, electrolysis,
   elision, ellipsis, enervation, enfeeblement, epitome, equation,
   evisceration, evolution, exhaustion, extenuation, extraction,
   extrapolation, fade-out, falling-off, fatigue, ferment,
   fermentation, flip-flop, foreshortening, growth, hauling down,
   hollowness, hourglass, hourglass figure, humbling, humiliation,
   hushing, hydrogenation, impairment, inanition, integration,
   interpolation, inversion, involution, isomerism, isthmus, kickback,
   knitting, languishment, lapse, leniency, lessening, letdown, letup,
   lightening, loosening, lowering, lulling, markdown, metamerism,
   metamerization, miniaturization, mitigation, modulation,
   mollification, multiplication, narrow place, narrowing,
   naturalization, neck, nitration, nose dive, notation, numbing,
   oxidation, oxidization, pacification, palliation, passage, penalty,
   penalty clause, percentage, phosphatization, plummet, plummeting,
   plunge, polymerism, polymerization, position isomerism, practice,
   precis, premium, price cut, price fall, price reduction, price-cut,
   progress, proportion, puckering, pursing, quelling, quietening,
   quieting, re-formation, rebate, rebatement, recap, recapitulation,
   reconversion, refund, relaxation, relief, remedy, remission,
   resolution, retraction, retrenchment, reversal, rollback, sag,
   sagging, salvage, salving, saturization, scaling down, setoff,
   shift, shortening, shrinkage, simplicity, sinking, slackening,
   slash, slump, softening, solidification, soothing, stranglement,
   strangulation, striction, stricture, stripping of rank, subdual,
   subduement, submergence, subtraction, summary, summation,
   suppression, switch, switch-over, syncope, synopsis, systole,
   taming, tare, telescoping, tempering, thinning, thrusting under,
   time discount, trade discount, tranquilization, transformation,
   transit, transition, treading down, tret, truncation, turning into,
   underselling, volte-face, wasp waist, weakening, working,
   wrinkling, write-off



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

reduction
contraction

   (Or "contraction") The process of transforming an expression
   according to certain reduction rules.  The most important
   forms are beta reduction (application of a lambda
   abstraction to one or more argument expressions) and delta
   reduction (application of a mathematical function to the
   required number of arguments).

   An evaluation strategy (or reduction strategy), determines
   which part of an expression (which redex) to reduce first.
   There are many such strategies.

   See graph reduction, string reduction, normal order
   reduction, applicative order reduction, parallel
   reduction, alpha conversion, beta conversion, delta
   conversion, eta conversion.

   (1995-02-21)