Mansions are especially large houses. Real estate agents in the US consider a mansion to be a residential house of over 8,000 square feet.
Traditional mansions in European countries are defined as houses that held a ballroom and several bedrooms. Modern architecture does not formally define the word beyond being recognized as an especially large and well-appointed house.
The word derives from the Latin word "mansiō which means "the act of remaining or staying. In Roman civilizations a mansion was an official place of stopping upon a road where the villas of official were placed and cities sprang up.
An "English Country House is a stylized type of mansion.
In older days it was popular among elitists of Europe to move from country home to country home, at which point unfortified castles and country houses began to evolve into more aesthetic homes. In the 1500s these mansions began to develop gracious styles with gardens, parks and drives. Up until the early 20th century it was not unusual for medium sized mansions in England to have both indoor and outdoor staffs of 20 people each, with larger mansions acquiring even more.