The Old Testament in Byzantium: Selected papers from a symposium held Dec. 2006, Dumbarton Oaks
The first recorded use of the word Ὀκτάτευχος (literally “eight books”) was by Prokopios of Gaza (d. 538), who called a volume of his biblical commentary Exegeses of the Octateuch (Εἰς τὴν Ὀκτάτευχον ἐξηγήσεις). The term is derived from the more commonly encountered term “Pentateuch.” The Octateuch is a unit commencing with the five books of Moses, generally known to both hellenized Jews and Greek-speaking Christians as the Law (ὁ Νόμος [Torah])—that is, Gen- esis, Exodus, Leviticus, Numbers, and Deuteronomy. To the Pentateuch were added the historical books of Joshua and Judges, which continue the narrative of Deuteronomy, and the short book of Ruth, which is set in the period of Judges (Ruth 1:1).
Although no manuscript of a complete single-volume Greek Octateuch survives from earlier than the tenth century,3 the special status of these biblical books in the Greek world is already implied in the fifth century. Theodoret of Cyrrhus (died ca. 466) wrote a commentary on the first eight books of the Old Testament, although he did not, as it happens, use the term Octateuch. Nevertheless, we can say with some confidence that the Octateuch as a distinct codex was an innovation of the fifth or sixth century, even though no examples from that era now survive.