Expensive Mansion, Toronto, Canada

The word itself derives (through Old French) from the Latin word mansiō ("act of remaining or staying", a verbal noun from manere "to remain" or "to stay".) In the Roman Empire, a mansio was an official stopping place on a Roman road, or via, where cities sprang up, and where the villas of provincial officials came to be placed. The Scots word "manse" originally defined a property large enough for the Minister of the parish to maintain himself, but a mansion is no longer self-sustaining in this way (compare a Roman or medieval villa). 'Manor' comes from the same root — territorial holdings granted to a lord who would remain there — hence it can be seen how the word 'Mansion' came to have its meaning.
History
The "country house," as it is known in English-speaking places, is a distinct variety of mansion.