A mansion is a large dwelling house.
The word itself derives (through Old French) from the Latin word mansio "dwelling", an abstract noun derived from the verb manere "to dwell". The English word "manse" originally defined a property large enough for the parish priest 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 is easy to see how the word "Mansion" came to have its meaning.