The Collaborative International Dictionary of English v.0.48:

Understand \Un`der*stand"\ ([u^]n`d[~e]r*st[a^]nd"), v. t. [imp.
   & p. p. Understood ([u^]n`d[~e]r*st[oo^]d"), and Archaic
   Understanded; p. pr. & vb. n. Understanding.] [OE.
   understanden, AS. understandan, literally, to stand under;
   cf. AS. forstandan to understand, G. verstehen. The
   development of sense is not clear. See Under, and Stand.]
   1. To have just and adequate ideas of; to apprehended the
      meaning or intention of; to have knowledge of; to
      comprehend; to know; as, to understand a problem in
      Euclid; to understand a proposition or a declaration; the
      court understands the advocate or his argument; to
      understand the sacred oracles; to understand a nod or a
      wink.
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            Speaketh [i. e., speak thou] so plain at this time,
            I you pray,
            That we may understande what ye say.  --Chaucer.
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            I understand not what you mean by this. --Shak.
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            Understood not all was but a show.    --Milton.
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            A tongue not understanded of the people. --Bk. of
                                                  Com. Prayer.
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   2. To be apprised, or have information, of; to learn; to be
      informed of; to hear; as, I understand that Congress has
      passed the bill.
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   3. To recognize or hold as being or signifying; to suppose to
      mean; to interpret; to explain.
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            The most learned interpreters understood the words
            of sin, and not of Abel.              --Locke.
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   4. To mean without expressing; to imply tacitly; to take for
      granted; to assume.
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            War, then, war,
            Open or understood, must be resolved. --Milton.
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   5. To stand under; to support. [Jocose & R.] --Shak.
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   To give one to understand, to cause one to know.

   To make one's self understood, to make one's meaning clear.
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The Collaborative International Dictionary of English v.0.48:

Understand \Un`der*stand"\, v. i.
   1. To have the use of the intellectual faculties; to be an
      intelligent being.
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            Imparadised in you, in whom alone
            I understand, and grow, and see.      --Donne.
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   2. To be informed; to have or receive knowledge.
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            I came to Jerusalem, and understood of the evil that
            Eliashib did for Tobiah.              --Neh. xiii.
                                                  7.
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WordNet (r) 3.0 (2006):

understand
    v 1: know and comprehend the nature or meaning of; "She did not
         understand her husband"; "I understand what she means"
    2: perceive (an idea or situation) mentally; "Now I see!"; "I
       just can't see your point"; "Does she realize how important
       this decision is?"; "I don't understand the idea" [syn:
       understand, realize, realise, see]
    3: make sense of a language; "She understands French"; "Can you
       read Greek?" [syn: understand, read, interpret,
       translate]
    4: believe to be the case; "I understand you have no previous
       experience?" [syn: understand, infer]
    5: be understanding of; "You don't need to explain--I
       understand!" [syn: sympathize, sympathise, empathize,
       empathise, understand]