The Collaborative International Dictionary of English v.0.48:

Deduction \De*duc"tion\, n. [L. deductio: cf. F. d['e]duction.]
   1. Act or process of deducing or inferring.
      [1913 Webster]

            The deduction of one language from another.
                                                  --Johnson.
      [1913 Webster]

            This process, by which from two statements we deduce
            a third, is called deduction.         --J. R. Seely.
      [1913 Webster]

   2. Act of deducting or taking away; subtraction; as, the
      deduction of the subtrahend from the minuend.
      [1913 Webster]

   3. That which is deduced or drawn from premises by a process
      of reasoning; an inference; a conclusion.
      [1913 Webster]

            Make fair deductions; see to what they mount.
                                                  --Pope.
      [1913 Webster]

   4. That which is or may be deducted; the part taken away;
      abatement; as, a deduction from the yearly rent in
      compensation for services; deductions from income in
      calculating income taxes.

   Syn: See Induction.
        [1913 Webster]

WordNet (r) 3.0 (2006):

deduction
    n 1: a reduction in the gross amount on which a tax is
         calculated; reduces taxes by the percentage fixed for the
         taxpayer's income bracket [syn: tax write-off, tax
         deduction, deduction]
    2: an amount or percentage deducted [syn: deduction,
       discount]
    3: something that is inferred (deduced or entailed or implied);
       "his resignation had political implications" [syn:
       deduction, entailment, implication]
    4: reasoning from the general to the particular (or from cause
       to effect) [syn: deduction, deductive reasoning,
       synthesis]
    5: the act of subtracting (removing a part from the whole); "he
       complained about the subtraction of money from their
       paychecks" [syn: subtraction, deduction] [ant:
       addition]
    6: the act of reducing the selling price of merchandise [syn:
       discount, price reduction, deduction]

Moby Thesaurus II by Grady Ward, 1.0:

175 Moby Thesaurus words for "deduction":
   Baconian method, a fortiori reasoning, a posteriori reasoning,
   a priori reasoning, abatement, ablation, abrasion, abridgment,
   abstraction, accord, addition, adjunct, affairs, affiliation,
   affinity, agio, alleviation, alliance, allowance, analysis,
   approximation, assemblage, association, attenuation, bank discount,
   bond, breakage, cash discount, chain discount, charge-off,
   closeness, cogitation, combination, concession, conclusion,
   connectedness, connection, consequence, consequent, consideration,
   contemplation, contiguity, contraction, contrariety, corollary,
   credit, cut, dampening, damping, dealings, decrease, decrement,
   decrescence, deductive reasoning, deflation, deliberation,
   demonstration, depreciation, depression, derivation, diminishment,
   diminution, discount, discourse, discourse of reason,
   discursive reason, disjunction, dockage, drawback, dying,
   dying off, epagoge, erosion, extenuation, fade-out, filiation,
   finding, generalization, homology, hypothesis and verification,
   illation, induction, inductive reasoning, inference, intercourse,
   intimacy, judgment, junction, kickback, languishment, lessening,
   letup, liaison, link, linkage, linking, logical thought, lowering,
   meditation, miniaturization, minus, mitigation, musing,
   mutual attraction, nearness, particularization, penalty,
   penalty clause, percentage, philosophical induction, philosophy,
   pondering, premium, price reduction, price-cut, proof, propinquity,
   proximity, purification, rapport, ratiocination, rationalism,
   rationality, rationalization, rationalizing, reason,
   reasonableness, reasoning, rebate, rebatement, reduction,
   refinement, reflection, refund, relatedness, relation, relations,
   relationship, relaxation, removal, result, rollback, rumination,
   sagging, salvage, scaling down, sequitur, setoff, similarity,
   simplicity, sophistry, specious reasoning, speculation, subduction,
   sublation, subtraction, sweet reason, syllogism,
   syllogistic reasoning, sympathy, synthesis, takeoff, taking away,
   tare, thinking, tie, tie-in, time discount, trade discount, tret,
   underselling, union, weakening, withdrawal, write-off