Casperia Prime is a world located within the Federation.
The world was mainly known as a tourism location and was referred to as the "vacation capital of the Horvian Cluster". (DS9: "Change of Heart")
The planet was also a premier location for conferences. Starfleet Medical held a conference there is 2374. (DS9: "Inquisition")
Casperia Prime article at Memory Alpha, the canon Star Trek wiki.