The Collaborative International Dictionary of English v.0.48:

Presumption \Pre*sump"tion\ (?; 215), n. [L. praesumptio: cf. F.
   pr['e]somption, OF. also presumpcion. See Presume.]
   1. The act of presuming, or believing upon probable evidence;
      the act of assuming or taking for granted; belief upon
      incomplete proof.
      [1913 Webster]

   2. Ground for presuming; evidence probable, but not
      conclusive; strong probability; reasonable supposition;
      as, the presumption is that an event has taken place.
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   3. That which is presumed or assumed; that which is supposed
      or believed to be real or true, on evidence that is
      probable but not conclusive. "In contradiction to these
      very plausible presumptions." --De Quincey.
      [1913 Webster]

   4. The act of venturing beyond due beyond due bounds; an
      overstepping of the bounds of reverence, respect, or
      courtesy; forward, overconfident, or arrogant opinion or
      conduct; presumptuousness; arrogance; effrontery.
      [1913 Webster]

            Thy son I killed for his presumption. --Shak.
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            I had the presumption to dedicate to you a very
            unfinished piece.                     --Dryden.
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   Conclusive presumption. See under Conclusive.

   Presumption of fact (Law), an argument of a fact from a
      fact; an inference as to the existence of one fact not
      certainly known, from the existence of some other fact
      known or proved, founded on a previous experience of their
      connection; supposition of the truth or real existence of
      something, without direct or positive proof of the fact,
      but grounded on circumstantial or probable evidence which
      entitles it to belief. --Burrill. --Best. --Wharton.

   Presumption of law (Law), a postulate applied in advance to
      all cases of a particular class; e. g., the presumption of
      innocence and of regularity of records. Such a presumption
      is rebuttable or irrebuttable.
      [1913 Webster]

WordNet (r) 3.0 (2006):

presumption
    n 1: an assumption that is taken for granted [syn: given,
         presumption, precondition]
    2: (law) an inference of the truth of a fact from other facts
       proved or admitted or judicially noticed
    3: audacious (even arrogant) behavior that you have no right to;
       "he despised them for their presumptuousness" [syn:
       presumption, presumptuousness, effrontery,
       assumption]
    4: a kind of discourtesy in the form of an act of presuming;
       "his presumption was intolerable"