The Septuagint
ἡ μετάφρασις τῶν Ἑβδομήκοντα — Septuaginta (LXX) · sep-TOO-ah-jint
In brief
The Septuagint is the ancient Greek translation of the Old Testament, made by Jewish scholars in Alexandria beginning in the third century before Christ. It was the Bible of the apostles — most Old Testament quotations in the New Testament follow its wording — and it remains the Old Testament as the Orthodox Church receives and uses it — the basis of her services and her wider canon — which is why Orthodox Bibles include books and passages that Protestant Bibles do not.
Where it came from
By the third century BC, more Jews lived in the Greek-speaking world than could comfortably read Hebrew, above all in Alexandria. There the Torah was translated into Greek — tradition says by about seventy scholars, from which the version takes its name (Septuaginta, "seventy," abbreviated LXX) — with the remaining books following over the next century or more. It was the first major translation of Scripture into another tongue, and it made Israel's God readable to the whole Mediterranean world.
When the apostles preached that world, this was the Bible in their hands. The New Testament quotes the Old roughly three hundred times, and more often than not the wording is the Septuagint's — including some famous places where it differs from the later standard Hebrew text. Matthew's "a virgin shall conceive" (Isaiah 7:14) follows the Septuagint's parthenos, "virgin." For the Church this is not an embarrassment but a providence: the translation itself belongs to the preparation of the Gospel.
Why the Orthodox Old Testament is bigger
The Septuagint carried a wider collection than the Hebrew canon later fixed by the rabbis: books like Tobit, Judith, Wisdom of Solomon, Sirach, Baruch, and the Maccabees, and longer texts of Daniel and Esther. The Orthodox Church received these — it calls them the Anagignoskomena, "things to be read" — and they remain in Orthodox Bibles and services. The Reformation, returning to the rabbinic Hebrew canon, set them aside, which is why a Protestant Old Testament is shorter than an Orthodox one.
The Orthodox Psalter is likewise the Septuagint Psalter, whose numbering differs by one from the Hebrew through most of the book — the "Psalm 50" of Orthodox prayer books is the "Psalm 51" of most English Bibles. Encyclopedia entries and HolyStudy generally give both numbers where confusion is possible.
The Septuagint in the Church's life
Orthodox services quote the Old Testament in its Septuagint form; the Fathers overwhelmingly commented on it; and the Church's doctrinal reading of key passages rests on it. This does not make the Hebrew text an enemy — Orthodox scholars use it constantly, and modern Orthodox translations consult both. But where the traditions diverge, the Church's instinct is to stand with the text the apostles quoted and the Fathers expounded. The Septuagint is less a scholarly artifact than a living inheritance: the Old Testament as the Church has always heard it.