BASILEUS, βασιλεύς, a title assumed by the emperors of Constantinople, exclusive of all other princes, to whom they give the title rex, "king." The same quality was afterwards given by them to the kings of Bulgaria, and to Charlemagne, from the successors of which last they endeavoured to wrest it back again.
The title basileus has been since assumed by other kings, particularly the kings of England, Ego Edgar totius Anglie basileus confirmavi. Hence also the queen of England was entitled Basilica and Basilissa.