The Collaborative International Dictionary of English v.0.48:

Assumption \As*sump"tion\ (?; 215), n. [OE. assumpcioun a taking
   up into heaven, L. assumptio a taking, fr. assumere: cf. F.
   assomption. See Assume.]
   1. The act of assuming, or taking to or upon one's self; the
      act of taking up or adopting.
      [1913 Webster]

            The assumption of authority.          --Whewell.
      [1913 Webster]

   2. The act of taking for granted, or supposing a thing
      without proof; supposition; unwarrantable claim.
      [1913 Webster]

            This gives no sanction to the unwarrantable
            assumption that the soul sleeps from the period of
            death to the resurrection of the body. --Thodey.
      [1913 Webster]

            That calm assumption of the virtues.  --W. Black.
      [1913 Webster]

   3. The thing supposed; a postulate, or proposition assumed; a
      supposition.
      [1913 Webster]

            Hold! says the Stoic; your assumption's wrong.
                                                  --Dryden.
      [1913 Webster]

   4. (Logic) The minor or second proposition in a categorical
      syllogism.
      [1913 Webster]

   5. The taking of a person up into heaven. Hence: (Rom. Cath.
      & Greek Churches) A festival in honor of the ascent of the
      Virgin Mary into heaven.
      [1913 Webster]

WordNet (r) 3.0 (2006):

assumption
    n 1: a statement that is assumed to be true and from which a
         conclusion can be drawn; "on the assumption that he has
         been injured we can infer that he will not to play" [syn:
         premise, premiss, assumption]
    2: a hypothesis that is taken for granted; "any society is built
       upon certain assumptions" [syn: assumption, supposition,
       supposal]
    3: the act of taking possession of or power over something; "his
       assumption of office coincided with the trouble in Cuba";
       "the Nazi assumption of power in 1934"; "he acquired all the
       company's assets for ten million dollars and the assumption
       of the company's debts" [syn: assumption, laying claim]
    4: celebration in the Roman Catholic Church of the Virgin Mary's
       being taken up into heaven when her earthly life ended;
       corresponds to the Dormition in the Eastern Orthodox Church
       [syn: Assumption, Assumption of Mary, August 15]
    5: (Christianity) the taking up of the body and soul of the
       Virgin Mary when her earthly life had ended
    6: audacious (even arrogant) behavior that you have no right to;
       "he despised them for their presumptuousness" [syn:
       presumption, presumptuousness, effrontery,
       assumption]
    7: the act of assuming or taking for granted; "your assumption
       that I would agree was unwarranted"