The Collaborative International Dictionary of English v.0.48:

mansion house \mansion house\ n.
   A large and imposing house.

   Syn: mansion, manse, hall, residence.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Mansion \Man"sion\, n. [OF. mansion, F. maison, fr. L. mansio a
   staying, remaining, a dwelling, habitation, fr. manere,
   mansum, to stay, dwell; akin to Gr. ?. Cf. Manse, Manor,
   Menagerie, Menial, Permanent.]
   1. A dwelling place, -- whether a part or whole of a house or
      other shelter. [Obs.]
      [1913 Webster]

            In my Father's house are many mansions. --John xiv.
                                                  2.
      [1913 Webster]

            These poets near our princes sleep,
            And in one grave their mansions keep. --Den?am.
      [1913 Webster]

   2. The house of the lord of a manor; a manor house; hence:
      Any house of considerable size or pretension.
      [1913 Webster]

   3. (Astrol.) A twelfth part of the heavens; a house. See 1st
      House, 8. --Chaucer.
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   4. The place in the heavens occupied each day by the moon in
      its monthly revolution. [Obs.]
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            The eight and twenty mansions
            That longen to the moon.              --Chaucer.
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   Mansion house, the house in which one resides;
      specifically, in London and some other cities, the
      official residence of the Lord Mayor. --Blackstone.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mansion house
    n 1: a large and imposing house [syn: mansion, mansion
         house, manse, hall, residence]