The Collaborative International Dictionary of English v.0.48:

Admittance \Ad*mit"tance\, n.
   1. The act of admitting.
      [1913 Webster]

   2. Permission to enter; the power or right of entrance; also,
      actual entrance; reception.
      [1913 Webster]

            To gain admittance into the house.    --South.
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            He desires admittance to the king.    --Dryden.
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            To give admittance to a thought of fear. --Shak.
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   3. Concession; admission; allowance; as, the admittance of an
      argument. [Obs.] --Sir T. Browne.
      [1913 Webster]

   4. Admissibility. [Obs.] --Shak.
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   5. (Eng. Law) The act of giving possession of a copyhold
      estate. --Bouvier.
      [1913 Webster]

   Syn: Admission; access; entrance; initiation.

   Usage: Admittance, Admission. These words are, to some
          extent, in a state of transition and change.
          Admittance is now chiefly confined to its primary
          sense of access into some locality or building. Thus
          we see on the doors of factories, shops, etc. "No
          admittance." Its secondary or moral sense, as
          "admittance to the church," is almost entirely laid
          aside. Admission has taken to itself the secondary or
          figurative senses; as, admission to the rights of
          citizenship; admission to the church; the admissions
          made by one of the parties in a dispute. And even when
          used in its primary sense, it is not identical with
          admittance. Thus, we speak of admission into a
          country, territory, and other larger localities, etc.,
          where admittance could not be used. So, when we speak
          of admission to a concert or other public assembly,
          the meaning is not perhaps exactly that of admittance,
          viz., access within the walls of the building, but
          rather a reception into the audience, or access to the
          performances. But the lines of distinction on this
          subject are one definitely drawn.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Admittance \Ad*mit"tance\, n. (Elec.)
   The reciprocal of impedance.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

admittance
    n 1: the right to enter [syn: entree, access, accession,
         admission, admittance]
    2: the act of admitting someone to enter; "the surgery was
       performed on his second admission to the clinic" [syn:
       admission, admittance]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

ADMITTANCE, Eng. law. The act of giving possession of a copyhold estate, as
livery of seisin is of a freehold; it is of three kinds, namely
upon a voluntary grant by the lord) upon a surrender by the former tenant and

upon descent.