The Collaborative International Dictionary of English v.0.48:

Dichotomy \Di*chot"o*my\, n. [Gr. ?, fr. ?: cf. F. dichotomie.
   See Dichotomous.]
   1. A cutting in two; a division.
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            A general breach or dichotomy with their church.
                                                  --Sir T.
                                                  Browne.
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   2. Division or distribution of genera into two species;
      division into two subordinate parts.
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   3. (Astron.) That phase of the moon in which it appears
      bisected, or shows only half its disk, as at the
      quadratures.
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   4. (Biol.) Successive division and subdivision, as of a stem
      of a plant or a vein of the body, into two parts as it
      proceeds from its origin; successive bifurcation.
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   5. The place where a stem or vein is forked.
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   6. (Logic) Division into two; especially, the division of a
      class into two subclasses opposed to each other by
      contradiction, as the division of the term man into white
      and not white.
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WordNet (r) 3.0 (2006):

dichotomy
    n 1: being twofold; a classification into two opposed parts or
         subclasses; "the dichotomy between eastern and western
         culture" [syn: dichotomy, duality]

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "dichotomy":
   Janus, abscission, ambiguity, ambivalence, amputation, bifidity,
   biformity, bifurcation, bipartition, bisection, branching,
   butchering, by two, chopping, cleavage, conjugation, cutting,
   cutting in two, dimidiation, division, doubleness, doublethink,
   doubling, dualism, duality, duplexity, duplication, duplicity,
   enucleation, equivocality, excision, fission, forking, halving,
   in half, irony, laceration, mutilation, pairing, polarity,
   ramification, rending, resection, ripping, scission, section,
   severance, slashing, slicing, splitting, subdivision, surgery,
   tearing, twinning, two-facedness, twoness