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]

Moby Thesaurus II by Grady Ward, 1.0:

120 Moby Thesaurus words for "understand":
   absorb, accept, agree, allow, appreciate, apprehend, arrange,
   assimilate, assume, be acquainted with, be afraid, be apprised of,
   be aware of, be cognizant of, be conscious of, be conversant with,
   be informed, be with one, believe, catch, catch on, catch on to,
   cognize, comprehend, conceit, conceive, conceptualize, conclude,
   conjecture, consider, construe, covenant, daresay, deduce, deem,
   define, describe, diagnose, dig, digest, discern, divine, dream,
   empathize with, expect, fancy, fathom, feel, follow, forgive,
   gather, get, get hold of, get it, get the drift, get the idea,
   get the picture, get wind of, grant, grasp, have,
   have information about, have it taped, have knowledge of, hear of,
   imagine, infer, interpret, ken, know, learn, let, let be, make out,
   master, opine, penetrate, perceive, perfume, possess, prefigure,
   prehend, presume, presuppose, presurmise, provisionally accept,
   read, read into, realize, reckon, recognize, repute, savvy, say,
   see, seize, seize the meaning, sense, suppose, surmise, suspect,
   sympathize with, take, take cognizance of, take for,
   take for granted, take in, take it, take it that, take to be,
   take to mean, think, tolerate, tumble to, twig, understand by,
   view, wit, wot, wot of