[8] The first written instance of this word is found on the baked clay tablets discovered in excavations of Mycenaean palaces originally destroyed by fire.

Otherwise the term survived almost exclusively as a component in compound personal names (e.g., Anaxagóras, Pleistoánax) and is still in use in Modern Greek in the description of the anáktoron / anáktora ("[place or home] of the ánax"), i.e. of the royal palace.

In Homer the wanax is expected to rule over the other basileis by consensus rather than by coercion, which is why Achilles rebels (the main theme of the Iliad) when he decides that Agamemnon is treating him disrespectfully.

A study by R. Drews[10] demonstrates that even at the apex of Geometric and Archaic Greek society, basileus did not automatically translate to "king": In a number of places authority was exercised by a college of basileis drawn from a particular clan or group, and the office had term limits.

Many Greek authors, reconciling Carthaginian supremacy in the western Mediterranean with eastern stereotypes of absolutist non-Hellenic government, termed the Punic chief magistrate, the sufet, as basileus in their native language.

[14] This conflation appears notably in Aristotle's otherwise positive description of the Carthaginian Constitution in the Politics, as well as in the writings of Polybius, Diodorus Siculus, and Diogenes Laertius.

It is at this time that the term basileus acquired a fully royal connotation, in stark contrast with the much less sophisticated earlier perceptions of kingship within Greece.

[18](pp 66–67) Nevertheless, despite its widespread use, due to its "royal" associations the title basileus remained unofficial for the Emperor, and was restricted in official documents to client kings in the East.

[18](p 70) The title appears to have slowly crept into imperial titulature after that, and Emperor Heraclius is attested as using it alongside the long-established Autokratōr Kaisar in a letter to Kavadh II in 628.

Finally, in a law promulgated on 21 March 629, the Latin titles were omitted altogether, and the simple formula πιστὸς ἐν Χριστῷ βασιλεύς, "faithful in Christ Emperor" was used instead.

[18](p 31) The adoption of the new imperial formula has been traditionally interpreted by scholars such as Ernst Stein and George Ostrogorsky as indicative of the almost complete Hellenization of the Empire by that time.

This usage was initially accepted by the "barbarian" kings of Western Europe themselves: Despite having neglected the fiction of Roman suzerainty from the 6th century onward, they refrained from adopting imperial titles.

The catalytic event was the coronation of Charlemagne as imperator Romanorum ("Emperor of the Romans") by Pope Leo III on 25 December 800, at St. Peter's in Rome.

The matter was complicated by the fact that the Eastern Empire was then managed by Irene (r. 797–802), who had gained control after the death of her husband, the Emperor Leo IV (r. 775–780), as regent for their nine-year-old son, Constantine VI (r. 780–797).

[19][b] The Pope would seize this opportunity to cite the imperial throne being held by a woman as vacant and establish his position as able to divinely appoint rulers.

[17](pp 263–264, 413) During the 12th century, Byzantine emperors of the Angelos dynasty, in their correspondence with the Pope and foreign rulers, styled themselves as "in Christ the God faithful, Emperor, crowned by God, Anax, the powerful, the exalted, ever Augustus, Autocrat of the Romans, the Angelos" (Medieval Greek: ἐν Χριστῷ τῷ Θεῷ πιστὸς βασιλεύς, θεοστεφής, ἄναξ, κραταιός, ὑψηλός, αὔγουστος καὶ αὐτοκράτωρ Ῥωμαίων, romanized: en Khristō to Theō pistos basileus, theostephēs, anax, krataios, hupsēlos, augoustos, kaì autokratōr Rhōmaiōn).

[21] By the Palaiologan period, the full style of the Emperor was finalized in the phrase, "in Christ the God faithful Emperor and Autocrat of the Romans, the Palaiologos" (Medieval Greek: ἐν Χριστῷ τῷ Θεῷ πιστὸς βασιλεὺς καὶ αὐτοκράτωρ Ῥωμαίων ὁ Παλαιολόγος, romanized: en Christō tō Theō pistós basileus kai autokratōr Rhōmaíōn ho Palaiológos), as exemplified in documents such as Constantine XI's chrysobull to the city of Ragusa issued in 1451, two years before the Ottoman conquest of the Byzantine Empire in the Siege of Constantinople.

[17](pp 263–264) A similar diplomatic controversy (this time accompanied by war) ensued from the imperial aspirations of Simeon I of Bulgaria in the early 10th century.

[17](pp 263–264) Finally, in 1354, Stefan Dušan, king of Serbia, assumed the imperial title, based on his Bulgarian mother's Theodora Smilets of Bulgaria royal line, self-styling himself in Greek as basileus and autokratōr of the Romans and Serbs which was, however, not recognized by the Byzantines.

During the post-Byzantine period, the term basileus, owing to the renewed influence of classical writers on the language, reverted to its earlier meaning of "king".

In the Convention of London in 1832, the Great Powers[c] agreed that the new Greek state should become a monarchy, and chose the Wittelsbach Prince Otto of Bavaria as its first king.

Second, that the kingship did not depend on the will of the Greek people, a fact further underlined by Otto's addition of the formula "ἐλέῳ Θεοῦ" eléo Theou, i.e. "By the Grace (Mercy) of God".