The Collaborative International Dictionary of English v.0.48:

Reciprocal \Re*cip"ro*cal\ (r[-e]*s[i^]p"r[-o]*kal), a. [L.
   reciprocus; of unknown origin.]
   1. Recurring in vicissitude; alternate.
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   2. Done by each to the other; interchanging or interchanged;
      given and received; due from each to each; mutual; as,
      reciprocal love; reciprocal duties.
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            Let our reciprocal vows be remembered. --Shak.
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   3. Mutually interchangeable.
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            These two rules will render a definition reciprocal
            with the thing defined.               --I. Watts.
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   4. (Gram.) Reflexive; -- applied to pronouns and verbs, but
      sometimes limited to such pronouns as express mutual
      action.
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   5. (Math.) Used to denote different kinds of mutual relation;
      often with reference to the substitution of reciprocals
      for given quantities. See the Phrases below.
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   Reciprocal equation (Math.), one which remains unchanged in
      form when the reciprocal of the unknown quantity is
      substituted for that quantity.

   Reciprocal figures (Geom.), two figures of the same kind
      (as triangles, parallelograms, prisms, etc.), so related
      that two sides of the one form the extremes of a
      proportion of which the means are the two corresponding
      sides of the other; in general, two figures so related
      that the first corresponds in some special way to the
      second, and the second corresponds in the same way to the
      first.

   Reciprocal proportion (Math.), a proportion such that, of
      four terms taken in order, the first has to the second the
      same ratio which the fourth has to the third, or the first
      has to the second the same ratio which the reciprocal of
      the third has to the reciprocal of the fourth. Thus, 2:5:
      :20:8 form a reciprocal proportion, because 2:5:
      :1/20:1/8.

   Reciprocal quantities (Math.), any two quantities which
      produce unity when multiplied together.

   Reciprocal ratio (Math.), the ratio between the reciprocals
      of two quantities; as, the reciprocal ratio of 4 to 9 is
      that of 1/4 to [frac19].

   Reciprocal terms (Logic), those terms which have the same
      signification, and, consequently, are convertible, and may
      be used for each other.
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   Syn: Mutual; alternate.

   Usage: Reciprocal, Mutual. The distinctive idea of mutual
          is, that the parties unite by interchange in the same
          act; as, a mutual covenant; mutual affection, etc. The
          distinctive idea of reciprocal is, that one party acts
          by way of return or response to something previously
          done by the other party; as, a reciprocal kindness;
          reciprocal reproaches, etc. Love is reciprocal when
          the previous affection of one party has drawn forth
          the attachment of the other. To make it mutual in the
          strictest sense, the two parties should have fallen in
          love at the same time; but as the result is the same,
          the two words are here used interchangeably. The
          ebbing and flowing of the tide is a case where the
          action is reciprocal, but not mutual.
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The Collaborative International Dictionary of English v.0.48:

Reciprocal \Re*cip"ro*cal\, n.
   1. That which is reciprocal to another thing.
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            Corruption is a reciprocal to generation. --Bacon.
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   2. (Arith. & Alg.) The quotient arising from dividing unity
      by any quantity; thus 1/4 is the reciprocal of 4; 1/(a +
      b) is the reciprocal of a + b. The reciprocal of a
      fraction is the fraction inverted, or the denominator
      divided by the numerator.
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WordNet (r) 3.0 (2006):

reciprocal
    adj 1: concerning each of two or more persons or things;
           especially given or done in return; "reciprocal aid";
           "reciprocal trade"; "mutual respect"; "reciprocal
           privileges at other clubs" [syn: reciprocal, mutual]
           [ant: nonreciprocal]
    2: of or relating to the multiplicative inverse of a quantity or
       function; "the reciprocal ratio of a:b is b:a"
    n 1: something (a term or expression or concept) that has a
         reciprocal relation to something else; "risk is the
         reciprocal of safety"
    2: (mathematics) one of a pair of numbers whose product is 1:
       the reciprocal of 2/3 is 3/2; the multiplicative inverse of 7
       is 1/7 [syn: multiplicative inverse, reciprocal]
    3: hybridization involving a pair of crosses that reverse the
       sexes associated with each genotype [syn: reciprocal cross,
       reciprocal]

Moby Thesaurus II by Grady Ward, 1.0:

199 Moby Thesaurus words for "reciprocal":
   algorismic, algorithmic, aliquot, ally, alter ego, alternate,
   analogon, analogous, analogue, answering, associate, associated,
   back-and-forth, beating, brother, cardinal, changeable, circling,
   close copy, close match, coacting, coactive, coadjutant,
   coadjuvant, coefficient, coequal, cognate, collaborative,
   collective, collectivist, collectivistic, collusive, combined,
   commensal, common, communal, communalist, communalistic, communist,
   communistic, communitarian, commutable, commutative, commutual,
   companion, compensatory, complement, complemental, complementary,
   concerted, concordant, concurrent, concurring, congenator,
   congener, conjoint, conniving, convertible, cooperant, cooperating,
   cooperative, coordinate, correlate, correlative, correspondent,
   corresponding, counterpart, cyclic, decimal, differential, digital,
   double, duplicate, ecumenic, epochal, equal, equalizing,
   equipollent, equivalent, even, every other, exchanged, exponential,
   fellow, figural, figurate, figurative, finite, fractional, general,
   give-and-take, harmonious, harmonized, homologous, image,
   imaginary, impair, impossible, in common, infinite, integral,
   interchangeable, interchanged, intermittent, irrational,
   isochronal, joint, kindred spirit, like, likeness, logarithmic,
   logometric, match, mate, measured, metronomic, mutual,
   near duplicate, negative, noncompetitive, numeral, numerary,
   numerative, numeric, obverse, odd, ordinal, oscillatory, pair,
   parallel, pendant, periodical, permutable, picture, popular,
   positive, possible, prime, public, pulsing, punitive, radical,
   rational, real, reciprocating, reciprocative, recompensing,
   recompensive, recurrent, recurring, reparative, restitutive,
   retaliative, retaliatory, retributive, retributory, returnable,
   rhythmic, rotary, seasonal, second self, seesaw, serial, shared,
   similitude, simulacrum, sine wave, sister, social, socialistic,
   societal, soul mate, standard, steady, submultiple, such, suchlike,
   surd, swapped, switched, symbiotic, synergetic, synergic,
   synergistic, tally, tantamount, the like of, the likes of,
   to-and-fro, traded, transcendental, transposed, twin,
   uncompetitive, undulant, undulatory, up-and-down, wavelike,
   wheeling