[MysticCat]

"Basileus" (bah-SILL- ee-us) basically means "king" in Greek; it was also a title used for Rulers in the Eastern Roman Empire. It comes from the same root as our modern English "basilica" (originally, a royal hall for holding court, later such a hall converted to a church or a church modeled after a basilica, still later a church having special status).

Without commenting at all (since I can't) on why some groups might have chosen to use the title "Basileus," I would note that it is not at all uncommon for GLOs to use -- publically or privately -- special titles for their officers, nor is it uncommon for those titles to be drawn from ancient Greek, Roman or similar sources.