A mansion is a huge, fancy house. Some mansions are eventually turned into museums where you can get a look at an old wealthy family's art and furniture collections.
It takes a lot of money and a staff of workers to live in and take care of a mansion. Historically, post-fifteenth century European noblemen lived in mansions that became the typical style of home for aristocrats who no longer required the safety of fortified castles. The word mansion comes from the Latin root mansionem, "a staying or a remaining," from the stem manere, "to stay."