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]