The Collaborative International Dictionary of English v.0.48:

Explanation \Ex`pla*na"tion\, n. [L. explanatio: cf. OF.
   esplanation.]
   1. The act of explaining, expounding, or interpreting; the
      act of clearing from obscurity and making intelligible;
      as, the explanation of a passage in Scripture, or of a
      contract or treaty.
      [1913 Webster]

   2. That which explains or makes clear; as, a satisfactory
      explanation.
      [1913 Webster]

   3. The meaning attributed to anything by one who explains it;
      definition; interpretation; sense.
      [1913 Webster]

            Different explanations [of the Trinity]. --Bp.
                                                  Burnet.
      [1913 Webster]

   4. A mutual exposition of terms, meaning, or motives, with a
      view to adjust a misunderstanding, and reconcile
      differences; reconciliation; agreement; as, to come to an
      explanation.

   Syn: Definition; description; explication; exposition;
        interpretation; detail. See Definition.
        [1913 Webster]

WordNet (r) 3.0 (2006):

explanation
    n 1: a statement that makes something comprehensible by
         describing the relevant structure or operation or
         circumstances etc.; "the explanation was very simple"; "I
         expected a brief account" [syn: explanation, account]
    2: thought that makes something comprehensible
    3: the act of explaining; making something plain or
       intelligible; "I heard his explanation of the accident"

Moby Thesaurus II by Grady Ward, 1.0:

126 Moby Thesaurus words for "explanation":
   abstraction, accomplishment, account, allegorization, analysis,
   answer, ascertainment, body of theory, case, case in point, cause,
   clarification, clearance, clearing, clearing up, commentary,
   construction, cracking, criticism, decipherment, decoding,
   definition, delineation, demonstration, demythologization,
   denouement, description, destigmatization, destigmatizing,
   determination, disclosure, disentanglement, doctrinairism,
   doctrinality, doctrinarity, editing, elucidation, emblem,
   emendation, end, end result, enlightenment, euhemerism, example,
   exculpation, excuse, exegesis, exemplar, exemplification,
   explication, exponent, expose, exposition, expounding, finding,
   finding-out, grounds, illumination, illustration, instance,
   interpretation, issue, justification, key, light, meaning,
   mere theory, motive, object lesson, outcome, pretense, pretext,
   purgation, purging, rational ground, rationale, rationalization,
   reason, reason why, rehabilitation, reinstatement,
   relevant instance, representative, resolution, resolving,
   restoration, result, riddling, signification, simplification,
   solution, solving, sorting out, speculation, stated cause, symbol,
   tentative explanation, the big idea, the idea, the whatfor,
   the wherefore, the why, theoretic, theoretical basis,
   theoretical structure, theoretics, theoria, theoric, theorization,
   theory, type, typical example, underlying reason, unified theory,
   unlocking, unraveling, unriddling, unscrambling, unspinning,
   untangling, untwisting, unweaving, upshot, vindication, working,
   working-out