Basileus (Greek: βασιλεύς) is a Greek word which means “king” or “monarch” of a country. The title is often used when referring to rulers of the Ancient Greek nations, and was used to refer to the Byzantine emperors. The title was also used to refer to kings of modern Greece. The word can also be used to refer to bishops of the Eastern Orthodox Church and Eastern Catholic Churches.
The feminine forms are basileia (βασίλεια), basilis (βασιλίς), basilissa (βασίλισσα), or the archaic basilinna (βασιλίννα).