Koine Greek included styles ranging from more conservative literary forms to the spoken vernaculars of the time. As the dominant language of the Byzantine Empire, it developed further into Medieval Greek, which then turned into Modern Greek. Koine remained the court language of the Byzantine Empire until its ending in 1453, while Medieval and eventually Modern Greek were everyday languages.
Literary Koine was the medium of much of post-classical Greek literary and scholarly writing, such as the works of Plutarch and Polybius. Koine is also the language of the Christian New Testament, of the Septuagint (the 3rd-century BC Greek translation of the Hebrew Bible), and of most early Christian theological writing by the Church Fathers. In this context, Koine Greek is also known as "Biblical", "New Testament", "ecclesiastical" or "patristic" Greek. It continues to be used as the liturgical language of services in the Greek Orthodox Church.
The English-language name Koine derives from the Koine Greek term ἡ κοινὴ διάλεκτος, "the common dialect". The Greek word koinē (κοινή) itself means "common". The word is pronounced //, // or // in US English and // in UK English. The pronunciation of the word in Koine itself gradually changed from (close to the Classical Attic pronunciation ) to (close to the Modern Greek ). In Greek, the language has been referred to as Ελληνιστική Κοινή, "Hellenistic Koiné", in the sense of "Hellenistic supraregional language").
Ancient scholars used the term koine in several different senses. Scholars such as Apollonius Dyscolus(2nd century AD) and Aelius Herodianus (2nd century AD) maintained the term Koine to refer to the Proto-Greek language, while others used it to refer to any vernacular form of Greek speech which differed somewhat from the literary language.
When Koine Greek became a language of literature by the 1st century BC, some people distinguished two forms of Koine: written (Greek) as the literary post-classical form (which should not be confused with Atticism), and vernacular as the day-to-day spoken form. Others chose to refer to Koine as the Alexandrian dialect (ἡ Ἀλεξανδρέων διάλεκτος) or the dialect of Alexandria, or even the universal dialect of its time. Modern classicists have often used the former sense.