'Mansion house' definitions:

Definition of 'mansion house'

noun

A large and imposing house [syn: mansion, mansion house, manse, hall, residence]

From: GCIDE

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. [1913 Webster]
4. The place in the heavens occupied each day by the moon in its monthly revolution. [Obs.] [1913 Webster]
The eight and twenty mansions That longen to the moon. --Chaucer. [1913 Webster]
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]

From: GCIDE