The Collaborative International Dictionary of English v.0.48:

Mansion \Man"sion\, v. i.
   To dwell; to reside. [Obs.] --Mede.
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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.]
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            In my Father's house are many mansions. --John xiv.
                                                  2.
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            These poets near our princes sleep,
            And in one grave their mansions keep. --Den?am.
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   2. The house of the lord of a manor; a manor house; hence:
      Any house of considerable size or pretension.
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   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
    n 1: (astrology) one of 12 equal areas into which the zodiac is
         divided [syn: sign of the zodiac, star sign, sign,
         mansion, house, planetary house]
    2: a large and imposing house [syn: mansion, mansion house,
       manse, hall, residence]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "mansion":
   aspect, astrodiagnosis, astrology, astromancy, castle, chateau,
   court, estate, genethliac astrology, genethliacism, genethliacs,
   genethlialogy, hall, horoscope, horoscopy, house, manor,
   mundane astrology, mundane house, nativity, natural astrology,
   palace, palais, palatial residence, palazzo, planetary house,
   stargazing, tower, villa, zodiac



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

MANSION. This term is synonymous with house. (q.v.) 1 Chit. Pr. 167; 2 T.
R. 502; 1 Tho. Co. Litt. 215, n. 35; 9 B. & C. 681; S. C. 17 E. C. L. R.
472, and the cases there cited; Com. Dig. Justices, P 5; 3 Serg. & Rawle,
199. A portion only of a building may come under the description of a
mansion-house. 1 Leach, 89, 428; 1 East, P. C. C. 15, s. 19. 2 Bouv. Inst.
n. 1571, note.