The two terms are interchangeable in many ways, however the usage of “Ghost” when referring to the Third Person of the Blessed Trinity has fallen from use in the last half-century or so for a variety of reasons. Included among those is the most likely: namely, the demonic or deathly connotations attached in recent decades with the word “ghost,” which comes from Old English and German. But then there’s the fact that the English word “Spirit,” which comes from the French, is a better translation of the Latin words Spiritus Sanctus. If used in the proper context, though, “Ghost” is certainly still permissible for use in private prayer.