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:

Understanding \Un`der*stand"ing\ ([u^]n`d[~e]r*st[a^]nd"[i^]ng),
   a.
   Knowing; intelligent; skillful; as, he is an understanding
   man.
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The Collaborative International Dictionary of English v.0.48:

Understanding \Un`der*stand"ing\, n.
   1. The act of one who understands a thing, in any sense of
      the verb; knowledge; discernment; comprehension;
      interpretation; explanation.
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   2. An agreement of opinion or feeling; adjustment of
      differences; harmony; anything mutually understood or
      agreed upon; as, to come to an understanding with another.
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            He hoped the loyalty of his subjects would concur
            with him in the preserving of a good understanding
            between him and his people.           --Clarendon.
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   3. The power to understand; the intellectual faculty; the
      intelligence; the rational powers collectively conceived
      an designated; the higher capacities of the intellect; the
      power to distinguish truth from falsehood, and to adapt
      means to ends.
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            But there is a spirit in man; and the inspiration of
            the Almighty giveth them understanding. --Job xxxii.
                                                  8.
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            The power of perception is that which we call the
            understanding. Perception, which we make the act of
            the understanding, is of three sorts: 1. The
            perception of ideas in our mind; 2. The perception
            of the signification of signs; 3. The perception of
            the connection or repugnancy, agreement or
            disagreement, that there is between any of our
            ideas. All these are attributed to the
            understanding, or perceptive power, though it be the
            two latter only that use allows us to say we
            understand.                           --Locke.
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            In its wider acceptation, understanding is the
            entire power of perceiving an conceiving, exclusive
            of the sensibility: the power of dealing with the
            impressions of sense, and composing them into
            wholes, according to a law of unity; and in its most
            comprehensive meaning it includes even simple
            apprehension.                         --Coleridge.
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   4. Specifically, the discursive faculty; the faculty of
      knowing by the medium or use of general conceptions or
      relations. In this sense it is contrasted with, and
      distinguished from, the reason.
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            I use the term understanding, not for the noetic
            faculty, intellect proper, or place of principles,
            but for the dianoetic or discursive faculty in its
            widest signification, for the faculty of relations
            or comparisons; and thus in the meaning in which
            "verstand" is now employed by the Germans. --Sir W.
                                                  Hamilton.
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   Syn: Sense; intelligence; perception. See Sense.
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WordNet (r) 3.0 (2006):

understanding
    adj 1: characterized by understanding based on comprehension and
           discernment and empathy; "an understanding friend"
    n 1: the cognitive condition of someone who understands; "he has
         virtually no understanding of social cause and effect"
         [syn: understanding, apprehension, discernment,
         savvy]
    2: the statement (oral or written) of an exchange of promises;
       "they had an agreement that they would not interfere in each
       other's business"; "there was an understanding between
       management and the workers" [syn: agreement,
       understanding]
    3: an inclination to support or be loyal to or to agree with an
       opinion; "his sympathies were always with the underdog"; "I
       knew I could count on his understanding" [syn: sympathy,
       understanding]
    4: the capacity for rational thought or inference or
       discrimination; "we are told that man is endowed with reason
       and capable of distinguishing good from evil" [syn: reason,
       understanding, intellect]

Moby Thesaurus II by Grady Ward, 1.0:

352 Moby Thesaurus words for "understanding":
   IQ, Spartan, Vernunft, abatement of differences, acceptation,
   accepting, acclamation, accommodation, accord, accordance,
   accordant, acute, adjustment, affinity, agape, agreeable, agreeing,
   agreement, agreement of all, akin, all-knowing, alliance, amicable,
   amity, apperception, apperceptive, appercipient, apprehending,
   apprehension, apprehensive, argute, armed with patience, armistice,
   arrangement, astute, at one, attuned, aware, awareness, bargain,
   binding agreement, bleeding, bond, bonds of harmony, brain, brains,
   bright, brotherly love, caliber, capacity, caritas, cartel,
   cement of friendship, charitable, charity, chorus, clairvoyance,
   clement, cogent, cognizant, collective agreement, command,
   commiserative, commitment, common assent, common consent,
   communion, community, community of interests, compact,
   compassionate, compatibility, compatible, composition,
   comprehending, comprehension, compromise, conception, conceptive,
   conceptual, conceptualization, concert, concession, concord,
   concordance, concordant, concordat, concurrence, condolent,
   congenial, congeniality, conscious, consensus, consensus gentium,
   consensus of opinion, consensus omnium, consent, consentaneity,
   consortium, contract, convention, cop-out, cordial understanding,
   correspondence, corresponding, covenant, covenant of salt, deal,
   deductive power, desertion of principle, dicker, discerning,
   discernment, disciplined, discourse of reason, discrimination,
   discursive, discursive reason, empathetic, empathic, empathy,
   employment contract, en rapport, endurant, engagement,
   enlightenment, entente, entente cordiale, esemplastic power,
   esprit, esprit de corps, estimation, evasion of responsibility,
   farseeing, farsighted, feeling of identity, fellow feeling,
   fellowship, fix on, forbearant, forbearing, foreknowledge,
   foreseeing, foresighted, forethoughted, forethoughtful,
   formal agreement, frictionless, frictionlessness,
   general acclamation, general agreement, general consent,
   general voice, gentle, give-and-take, giving way, good sense,
   good vibes, good vibrations, grasp, gray matter, grip,
   happy family, harmonious, harmony, head, headpiece, human, humane,
   ideation, ideational, identity, import, in accord, in concert,
   in rapport, in tune, incisive, indulgent, inharmony, insight,
   insightful, integrative power, intellect, intellection,
   intellectual, intellectual faculty, intellectual grasp,
   intellectual power, intellectualism, intellectuality, intelligence,
   intelligence quotient, intelligent, intendment, interpretation,
   intuition, ironclad agreement, judgement, kinship, knowing,
   knowledge, knowledgeable, legal agreement, legal contract, lenient,
   like-minded, like-mindedness, long-suffering, longheaded,
   longsighted, love, mastery, meaning, meeting of minds, melting,
   mens, mental age, mental capacity, mental grasp, mental ratio,
   mentality, merciful, message, mind, mindful, mother wit,
   mutual agreement, mutual concession, mutual understanding,
   mutuality, native wit, noetic, not so dumb, notion, nous,
   obligation, of one mind, omniscient, one accord, one voice,
   oneness, opinion, pact, paction, patient, patient as Job, peace,
   peaceful, penetrating, penetration, perception, perceptive,
   percipience, percipient, persevering, perspicacious, perspicuous,
   philosophical, piercing, pitying, power of mind, power of reason,
   precognition, preengagement, prehensile, prehension, promise,
   protocol, provident, psyche, purport, rapport, rapprochement,
   ratio, rational, rationality, reading, reason, reasonable,
   reasoning, reasoning faculty, reasoning power, reciprocity,
   recognizance, reconciliation, ruthful, sagacious, sagacity,
   sageness, same mind, sane, sanity, sapience, savvy, scope of mind,
   self-controlled, sense, sensible, sensitiveness, sensitivity,
   settlement, sharing, shrewd, significance, significancy,
   signification, single voice, smarts, soft, softhearted, solidarity,
   sophic, stipulation, stoic, strong-minded, surrender, sympathetic,
   sympathizing, sympathy, symphony, team spirit, tender,
   tenderhearted, thinking power, together, tolerant, tolerating,
   tolerative, total agreement, transaction, treaty, trenchant, truce,
   unanimity, unanimousness, undertaking, union, union contract,
   unison, united, unity, universal agreement, valid contract,
   verbal agreement, view, wage contract, warmhearted, wisdom, wise,
   wit, yielding



The Devil's Dictionary (1881-1906):

UNDERSTANDING, n.  A cerebral secretion that enables one having it to
know a house from a horse by the roof on the house.  Its nature and
laws have been exhaustively expounded by Locke, who rode a house, and
Kant, who lived in a horse.

    His understanding was so keen
    That all things which he'd felt, heard, seen,
    He could interpret without fail
    If he was in or out of jail.
    He wrote at Inspiration's call
    Deep disquisitions on them all,
    Then, pent at last in an asylum,
    Performed the service to compile 'em.
    So great a writer, all men swore,
    They never had not read before.
                                                       Jorrock Wormley