Most previous Tales of games had a recognizable base word. Graces, Hearts, Destiny, Symphonia, Innocence, Phantasia, etc. Recently they've been using proper nouns as titles, Xillia and Vesperia for example.
In the context of the game, Vesperia is "the brightest star in the sky".
And Xillia is apparently an offshoot of Xillion which is a play on Zillion which means "countless" and the letter "X" being used to represent "the unknown"... so countless unknown things? I dunno where Baba gets this, tbh.
If they do Catastoria instead of Catastora, I can see that sounding good. :x
According to producer Baba, the name Xillia arranged in English expresses the word Xillion which in turn means countless. For our case, Xillion is a play on words of Zillion. The letter X signifies the meaning of the unknown, or crossing. Ones life can accumulate with countless things, good or bad.