Ancient Greek is the historical stage in the development of the Greek language spanning across the Archaic (c. 9th–6th centuries BC), Classical (c. 5th–4th centuries BC), and Hellenistic (c. 3rd century BC–6th century AD) periods of ancient Greece and the ancient world. It is predated in the 2nd millennium BC by Mycenaean Greek. Its Hellenistic phase is known as Koine ("common") or Biblical Greek, and its late period mutates imperceptibly into Medieval Greek. Koine is regarded as a separate historical stage of its own, although in its earlier form it closely resembles Classical Greek. Prior to the Koine period, Greek of the classic and earlier periods included several regional dialects.

The Ancient Greek language is one of the most prominent in human cultural history, as it was the language of the works of Homer, of the historians, playwrights and philosophers during the Athenian Golden Age, and of the New Testament. It has made a large contribution to the vocabulary of English, including such essential terms as democracy and was a standard subject of study in Western educational institutions from the Renaissance to the early 20th century. The New Latin used in the scientific binomial classification system continues today to draw vigorously from Ancient Greek vocabulary.

This article treats primarily the Archaic and Classical phases of the language — see also the article on Koine Greek.

Dialects of Ancient Greek

main: Ancient Greek dialects The origins, early forms, and early development of the Hellenic language family are not well understood, owing to the lack of contemporaneous evidence. There are several theories about what Hellenic dialect groups may have existed between the divergence of early Greek-like speech from the common Indo-European language (not later than 2000 BC), and about 1200 BC. They have the same general outline but differ in some of the detail. The only attested dialect from this period is Mycenaean, but its relationship to the historical dialects and the historical circumstances of the times imply that the overall groups already existed in some form.

The major dialect groups of the Ancient Greek period can be assumed to have developed not later than 1120 BC, at the time of the Dorian invasion(s), and their first appearances as precise alphabetic writing began in the 8th century BC. The invasion would not be "Dorian" unless the invaders had some cultural relationship to the historical Dorians; moreover, the invasion is known to have displaced population to the later Attic-Ionic regions, who regarded themselves as descendants of the population displaced by or contending with the Dorians.

The ancient Greeks themselves considered there to be three major divisions of the Greek people—Dorians, Aeolians, and Ionians (including Athenians), each with their own defining and distinctive dialects. Allowing for their oversight of Arcadian, an obscure mountain dialect, and Cyprian, far from the center of Greek scholarship, this division of people and language is quite similar to the results of modern archaeological-linguistic investigation.