Quote Originally Posted by Kawaii Ryukishi View Post
It's called a motif, generally. Every Final Fantasy since FFIV has featured similar rearrangements of its main theme.

My favorite arrangements of this one are the title screen track, the Oeilvert track (just because it's the first version you hear in the game itself, and it's far along enough in the game to surprise you with the connection it makes), and the serene theme of Terra.
This, exactly. The only thing I'll add is that there is more than one musical theme in this game that is reintroduced in an arrangement later on. Without these arrangements I don't think FFIX would have been half as stellar as it is.