I've always thought it was an attached, useful greenhouse, more of an inside tropical room with comfy furniture in which one may languish away the hours while listening to the humidity slowly slide off the fronds teasing the koi into snapping at the water in the fond with a quietly frolicking water fountain. There may also be canaries or other songbirds.

Mark Twain's house in West Hartford, CT has a small one off his library, it is so lovely, just beautiful and very inviting.