Koine Greek (Ελληνιστική Κοινή) “Hellenistic common (language)”; or ἡ κοινὴ διάλεκτος, “the common dialect” is the popular form of the Greek language spoken throughout post-Classical antiquity (c.300 BC – 300 A.D.), developing from the Attic dialect, with admixture of elements especially from Ionic. Koine Greek arose as a common dialect within the armies of Alexander the Great. Under the leadership of Macedon, which colonized the known world, their newly formed common dialect was spoken from Egypt to Mesopotamia. Though elements of Koine Greek took shape during the Classical Era, the post-Classical period of Greek is defined as beginning with the death of Alexander the Great in 323 B.C., when cultures under Hellenistic sway in turn began to influence the language. The passage into the next period, known as Medieval Greek, dates from the foundation of Constantinople by Constantine I in 330 A.D. The post-Classical period of Greek thus refers to the creation and evolution of Koine Greek throughout the entire Hellenistic and Roman eras of history until the start of the Middle Ages.
Κοινή is a term which had been previously applied by ancient scholars to several forms of Greek speech. A school of scholars such as Apollonius Dyscolus and Aelius Herodianus maintained the term Koine to refer to the Proto-Greek language, while others would use it to refer to any vernacular form of Greek speech which differed from the literary language. When Koine gradually became a language of literature, some people distinguished it in two forms: Hellenic (Greek) as the literary post-classical form, and Koine (common) as the spoken popular form. Others chose to refer to Koine as the Alexandrian dialect (Περὶ τῆς Ἀλεξανδρέων διαλέκτου) or the dialect of Alexandria, a term often used by modern classicists.
The linguistic roots of the Common Greek dialect had been unclear since ancient times. During the Hellenistic age, most scholars thought of Koine as the result of the mixture of the four main Ancient Greek dialects, “ἡ ἐκ τῶν τεττάρων συνεστῶσα” (the composition of the Four). Koine Greek can be regarded as Attic with the admixture of elements especially from Ionic, but also from other dialects. The degree of importance of the non-Attic linguistic elements on Koine can vary depending on the region of the Hellenistic World. In that respect, the varieties of Koine spoken in the Ionian colonies of Asia Minor would have more intense Ionic characteristics than others and those of Laconia and Cyprus would preserve some Doric and Arcado-Cypriot characteristics, respectively etc. The literary Koine of the Hellenistic age resembles Attic in such a degree that it is often mentioned as Common Attic.
Κοινή is also the language of the Septuagint and of the New Testament. As the language of the New Testament and of the Church Fathers, Koine Greek is also known as Biblical Greek, Patristic Greek or New Testament Greek.