The Collaborative International Dictionary of English v.0.48:

Transition \Tran*si"tion\, n. [L. transitio: cf. F. transition.
   See Transient.]
   1. Passage from one place or state to another; charge; as,
      the transition of the weather from hot to cold.
      [1913 Webster]

            There is no death, what seems so is transition.
                                                  --Longfellow.
      [1913 Webster]

   2. (Mus.) A direct or indirect passing from one key to
      another; a modulation.
      [1913 Webster]

   3. (Rhet.) A passing from one subject to another.
      [1913 Webster]

            [He] with transition sweet, new speech resumes.
                                                  --Milton.
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   4. (Biol.) Change from one form to another.
      [1913 Webster]

   Note: This word is sometimes pronounced tran*sish"un; but
         according to Walker, Smart, and most other authorities,
         the customary and preferable pronunciation is
         tran*sizh"un, although this latter mode violates
         analogy. Other authorities say tran*zish"un.
         [1913 Webster]

   Transition rocks (Geol.), a term formerly applied to the
      lowest uncrystalline stratified rocks (graywacke) supposed
      to contain no fossils, and so called because thought to
      have been formed when the earth was passing from an
      uninhabitable to a habitable state.
      [1913 Webster]

WordNet (r) 3.0 (2006):

transition
    n 1: the act of passing from one state or place to the next
         [syn: passage, transition]
    2: an event that results in a transformation [syn: conversion,
       transition, changeover]
    3: a change from one place or state or subject or stage to
       another
    4: a musical passage moving from one key to another [syn:
       transition, modulation]
    5: a passage that connects a topic to one that follows
    v 1: cause to convert or undergo a transition; "the company had
         to transition the old practices to modern technology"
    2: make or undergo a transition (from one state or system to
       another); "The airline transitioned to more fuel-efficient
       jets"; "The adagio transitioned into an allegro"

Moby Thesaurus II by Grady Ward, 1.0:

128 Moby Thesaurus words for "transition":
   about-face, accommodation, adaptation, adjustment, alchemy,
   alteration, amelioration, apostasy, assimilation, assumption,
   becoming, betterment, break, change, change of heart, change-over,
   changeableness, communication, conduction, constructive change,
   contagion, continuity, convection, conversion, defection,
   degeneration, degenerative change, delivery, deportation,
   deterioration, development, deviation, diapedesis, difference,
   diffusion, discontinuity, dissemination, divergence,
   diversification, diversion, diversity, evolution, export,
   exportation, expulsion, extradition, fitting, flip-flop,
   gradual change, growth, import, importation, improvement,
   interchange, lapse, melioration, metamorphosis, metastasis,
   metathesis, metempsychosis, migration, mitigation, modification,
   modulation, mutation, mutual transfer, naturalization, osmosis,
   overthrow, passage, passing over, perfusion, progress,
   qualification, radical change, re-creation, re-formation,
   realignment, reconversion, redesign, reduction, reform,
   reformation, remaking, renewal, reshaping, resolution,
   restructuring, reversal, revival, revivification, revolution,
   shift, spread, spreading, sudden change, switch, switch-over,
   total change, transduction, transfer, transfer of property,
   transference, transformation, transfusion, transit, translation,
   translocation, transmigration, transmigration of souls,
   transmission, transmittal, transmittance, transmutation,
   transplacement, transplantation, transposal, transposition, travel,
   turn, turnabout, turning into, upheaval, variation, variety,
   violent change, volte-face, worsening