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:

Understood \Un`der*stood"\,
   imp. & p. p. of Understand.
   [1913 Webster]

WordNet (r) 3.0 (2006):

understood
    adj 1: fully apprehended as to purport or meaning or
           explanation; "the understood conditions of troop
           withdrawal were clear" [ant: ununderstood]
    2: implied by or inferred from actions or statements; "gave
       silent consent"; "a tacit agreement"; "the understood
       provisos of a custody agreement" [syn: silent, tacit,
       understood]

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "understood":
   accepted, accounted as, acknowledged, admitted, agreed, alleged,
   appreciated, apprehended, arranged, ascertained, assumed,
   assumptive, comprehended, conceded, conceived, conjectured,
   conventional, covenanted, customary, deemed, discerned, down pat,
   established, fixed, folk, given, granted, grasped, hallowed,
   handed down, heroic, hoary, immemorial, implicit, implied,
   inferred, inveterate, known, legendary, long-established,
   long-standing, mythological, of long standing, of the folk, oral,
   pat, perceived, postulated, postulational, prehended, premised,
   prescriptive, presumed, presumptive, putative, realized, received,
   recognized, reputed, rooted, seized, settled, supposed,
   suppositional, supposititious, suppositive, tacit,
   taken for granted, time-honored, traditional, tried and true,
   true-blue, undeclared, unexpressed, unsaid, unspoken, unuttered,
   unwritten, venerable, wordless, worshipful