The feminine forms are basileia (βασίλεια), basilis (βασιλίς), basilissa (βασίλισσα), or the archaic basilinna (βασιλίννα), meaning "queen" or "empress".
The etymology of basileus is unclear. The Mycenaean form was *gʷasileus (Linear B: 𐀣𐀯𐀩𐀄, qa-si-re-u), denoting some sort of court official or local chieftain, but not an actual king. Its hypothetical earlier Proto-Greek form would be *gʷatileus. Most linguists assume that it is a non-Greek word that was adopted by Bronze Age Greeks from a pre-existing linguistic Pre-Greek substrate of the Eastern Mediterranean. Schindler (1976) argues for an inner-Greek innovation of the -eus inflection type from Indo-European material rather than a Mediterranean loan.
The first written instance of this word is found on the baked clay tablets discovered in excavations of Mycenaean palaces originally destroyed by fire. The tablets are dated from the 15th century BC to the 11th century BC and are inscribed with the Linear B script, which was deciphered by Michael Ventris in 1952 and corresponds to a very early form of Greek. The word basileus is written as qa-si-re-u and its original meaning was "chieftain" (in one particular tablet the chieftain of the guild of bronzesmiths is referred to as qa-si-re-u). Here the initial letter q- represents the PIE labiovelar consonant */gʷ/, transformed in later Greek into /b/. Linear B uses the same glyph for /l/ and /r/, now uniformly written with a Latin "r" by convention. Linear B only depicts syllables of single vowel or consonant-vowel form, therefore the final -s is dropped altogether.
The word can be contrasted with wanax, another word used more specifically for "king" and usually meaning "High King" or "overlord". With the collapse of Mycenaean society, the position of wanax ceases to be mentioned, and the basileis (the plural form) appear the topmost potentates in Greek society. In the works of Homer wanax appears, in the form ánax, mostly in descriptions of Zeus (ánax andrōn te theōn te, "king of men and of the gods") and of very few human monarchs, most notably Agamemnon. 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 (" of the ánax"), i.e. of the royal palace. The latter is essentially the same word as 𐀷𐀩𐀏𐀳𐀫 wa-na-ka-te-ro, wanákteros, "of the wanax/king" or "belonging to the wanax/king", used in Linear B tablets to refer to various craftsmen serving the king (e.g. the "palace", or royal, spinner, or the ivory worker), and to things belonging or offered to the king (javelin shafts, wheat, spices, precincts etc.).
Most of the Greek leaders in Homer's works are described as basileís, which is conventionally rendered in English as "kings". However, a more accurate translation may be "princes" or "chieftains", which would better reflect conditions in Greek society in Homer's time, and also the roles ascribed to Homer's characters. Agamemnon tries to give orders to Achilles among many others, while another basileus serves as his charioteer. His will, however, is not to be automatically obeyed. In Homer the wanax is expected to rule over the other basileis by consensus rather than by coercion, which is why Achilles proudly and furiously rebels (the central theme of the Iliad) when he perceives that Agamemnon is unjustly bossing him around.