Mugen is the word for "limitless" or "infinite" in Japanese.
Also "dream" or "phantom".
Well, that depends on how you write it. Could also be "six illusions".
But I'm pretty sure the word for "Phantom" is readed as "Gen'ei". "Mu" is a character that denotes void or absence of something. Perhaps if you used the "gen" of "genjitsu" (reality) it might apply, but it's a pretty awkward reading... Where have you seen that, Prime?
I'm gonna give people more time to react to the newly-introduced Mugen.