The Septuagint (often abbreviated to LXX) is an ancient Jewish translation of the Old Testament (and a few other Hebrew writings) into Greek. It was completed c. 250 BC in the city of Alexandria, Egypt. It is often the versions of the Old Testament that is quoted in the New Testament.
The name “Septuagint” is from from the Latin phrase Interpretatio septuaginta virorum meaning “translation of the seventy interpreters” and refers to the seventy Jewish scholars who were supposedly asked by Ptolemy II, the Greek King of Egypt, to translate the Hebrew Scriptures into Greek for the library of Alexandria.
For further information, see the Wikipedia entry on Septuagint.