1 a large house.
2 (usu. in pl.) Brit. a large building divided into flats.
Phrases and idioms:
mansion-house Brit. the house of a lord mayor or a landed proprietor. the Mansion House the official residence of the Lord Mayor of London.
Etymology: ME f. OF f. L mansio -onis a staying (as MANSE)
Useful english dictionary. 2012.