The ancient Greek translation of the Hebrew Old Testament, produced in Alexandria, Egypt, between the third and second centuries BC. The name (from Latin septuaginta, "seventy") derives from the legend in the Letter of Aristeas that 70 (or 72) Jewish elders translated the Torah in 70 days. The translation happened in stages: the Pentateuch first (c. 250 BC), then the Prophets and Writings in waves over the next century. By the first century AD the LXX was the functional Bible of most diaspora Jews and, soon, of the Gentile Christian Church.
The Septuagint is one of the most important non-biblical books for understanding the NT. Five observations. (1) The Bible of the early Church. Most NT authors quote the OT in Greek, and where the Hebrew and Greek differ, they often follow the LXX. Matthew 1:23's "virgin shall conceive" follows LXX Isaiah 7:14 (parthenos) rather than the Hebrew almah ("young woman") — a key Christological prophecy that the LXX already translated with specificity. (2) Bridge for the Gentile mission. Greek-speaking God-fearers in the synagogues of the diaspora already knew the LXX; Paul could preach the fulfilled Messiah using the same text they had been reading. (3) Textual witness. Where the LXX and MT differ, scholars have to ask which reflects the older Hebrew original. The Dead Sea Scrolls sometimes side with the LXX, indicating that the LXX preserves real alternate Hebrew readings, not merely loose translation. (4) Apocrypha. The LXX included books later excluded from the Protestant Old Testament (Tobit, Judith, Wisdom, Sirach, 1-4 Maccabees, etc.). Jerome's Vulgate preserved these as "deuterocanonical"; the Reformation rejected them as non-canonical but sometimes profitable to read. (5) Longer and different. The LXX differs in significant ways from the MT — the LXX Jeremiah is about 15% shorter; the LXX Daniel includes the Greek additions (Song of the Three Young Men, Susanna, Bel and the Dragon). Serious OT students will study both the MT and the LXX.