What is Basileus?

Legal Definition
Basileus (Greek: βασιλεύς) is a Greek term and title that has signified various types of monarchs in history. In the English-speaking world it is perhaps most widely understood to mean "king" or "emperor". The best known use of the title is by the Byzantine emperors, but the title also has a longer history of use by sovereigns and other persons of authority in ancient Greece, as well as for the kings of modern Greece.

The feminine forms are basilissa (Βασίλισσα), basileia (Βασίλεια), basilis (Βασιλίς), or the archaic basilinna (Βασιλίννα), meaning "queen" or "empress".
-- Wikipedia
Legal Definition
A Greek word, meaning "king " A title assumed by the emperors of the Eastern Roman Empire. It is used by Justinian in some of the Novels; and is said to have been applied to the English kings before the Conquest. See 1 Bl. Comm. 242.
-- Black's Law Dictionary
Legal Definition
A king. See 1 Bl. Comm. 242.
-- Ballentine's Law Dictionary