Arda is the world in which Middle Earth exists. Originally flat, it was converted to a sphere when Aman was separated from the rest of Arda.
Painting based on Karen Wynn Fonstad’s Atlas
Further Reading:
The ever excellent “The One Wiki to Rule Them All” has an entry on Arda
Wikipedia of course: https://en.wikipedia.org/wiki/Arda_(Tolkien)
If you want to see some more great maps for Tolkien or other Fantasy series, you should look up Karen Wynn Fonstad’s work.
Picture Credit: https://insholent.deviantart.com/art/Second-Age-of-Arda-436670630