Basileus (Ancient Greek: βασιλεύς) is a Greek term and title that has signified various types of monarchs throughout history. In the English-speaking world, it is perhaps most widely understood to mean 'monarch', referring to either a 'king' or an 'emperor'. The title was used by sovereigns and other persons of authority in ancient Greece (especially during the Hellenistic period), the Byzantine emperors, and the kings of modern Greece. The name Basileios (Basil), deriving from the term basileus, is a common given name in the Eastern Orthodox Church and Syriac Orthodox Church for the Maphrian.

The feminine forms are basileia (βασίλεια), basilissa (βασίλισσα), basillis (βασιλίς), or the archaic basilinna (βασιλίννα), meaning 'queen' or 'empress'. The related term basileia (βασιλεία) has meanings such as 'sovereignty', 'royalty', 'kingdom', 'reign', 'dominion' and 'authority'.

The etymology of basileus is uncertain. 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. Some 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 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 BCE to the 11th century BCE 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 transcribed with a Latin "r" by uniform convention. (Similarly, the Old Persian word vazir also has almost the same meaning as "chieftain".) Linear B only represents syllables of single vowel, or of a consonant-vowel form, therefore any final -s is omitted.

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 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 ("[place or home] 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 items 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 rendered conventionally in English as "kings". However, a more accurate translation may be "princes" or "chieftains", which would better represent 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 obeyed automatically. 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 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. However, basileus could also be applied to the hereditary leaders of "tribal" states, like those of the Arcadians and the Messenians, in which cases the term approximated the meaning of "king".

According to pseudo-Archytas's treatise "On justice and law" Basileus is more adequately translated into "Sovereign" than into "king". The reason for this is that it designates more the person of king than the office of king: the power of magistrates (arkhontes, "archons") derives from their social functions or offices, whereas the sovereign derives his power from himself. Sovereigns have auctoritas, whereas magistrates retain imperium. Pseudo-Archytas aimed at creating a theory of sovereignty completely enfranchised from laws, being itself the only source of legitimacy. He goes so far as qualifying the Basileus as nomos empsykhos, or "living law", which is the origin, according to Agamben, of the Führerprinzip and of Carl Schmitt's theories on dictatorship.