The Collaborative International Dictionary of English v.0.48:

Explain \Ex*plain"\ ([e^]ks*pl[=a]n"), v. t. [imp. & p. p.
   Explained([e^]ks*pl[=a]nd"); p. pr. & vb. n. Explaining.]
   [L. explandare to flatten, spread out, explain; ex out +
   plandare to make level or plain, planus plain: cf. OF.
   esplaner, explaner. See Plain,a., and cf. Esplanade.]
   1. To flatten; to spread out; to unfold; to expand. [Obs.]
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            The horse-chestnut is . . . ready to explain its
            leaf.                                 --Evelyn.
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   2. To make plain, manifest, or intelligible; to clear of
      obscurity; to expound; to unfold and illustrate the
      meaning of; as, to explain a chapter of the Bible.
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            Commentators to explain the difficult passages to
            you.                                  --Gay.
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   To explain away, to get rid of by explanation. "Those
      explain the meaning quite away." --Pope.

   Syn: To expound; interpret; elucidate; clear up.
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The Collaborative International Dictionary of English v.0.48:

Explain \Ex*plain"\, v. i.
   To give an explanation.
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WordNet (r) 3.0 (2006):

explain
    v 1: make plain and comprehensible; "He explained the laws of
         physics to his students" [syn: explain, explicate]
    2: define; "The committee explained their plan for fund-raising
       to the Dean"
    3: serve as a reason or cause or justification of; "Your need to
       sleep late does not excuse your late arrival at work"; "Her
       recent divorce may explain her reluctance to date again"
       [syn: excuse, explain]