(astrology) one of 12 equal areas into which the zodiac is divided
"In astrology, each sign occupies a mansion along the ecliptic path."
a large and imposing house
"The billionaire moved into his new mansion, which loomed over the quiet neighborhood with its towering columns and sprawling gardens."
A large house or building, usually built for the wealthy.
"The billionaire purchased a sprawling mansion overlooking the ocean."
In plain English: A mansion is an extremely large and expensive house that usually has many rooms, gardens, and special features for very wealthy people to live in.
"The wealthy family moved into their new mansion in the hills."
Usage: Use "mansion" to describe an exceptionally large and luxurious home owned by someone very wealthy, distinguishing it from simply calling any big house a "large estate." Avoid using this term unless you want to emphasize opulence and high status rather than just physical size.
The word "mansion" comes from the Latin mansio, which originally meant a dwelling or place where one stays. It entered English through Anglo-Norman and Middle French before evolving to describe a large house.