Definition from Wiktionary, the free dictionary
From Latin septuāgintā (“the seventy”), for the reputed 70 scholars who did the work.
Proper noun 
- An ancient translation of the Hebrew Bible into Greek, undertaken by Jews resident in Alexandria for the benefit of Jews who had forgotten their Hebrew (well before the birth of Jesus); abbreviated as LXX. The LXX is the untranslated standard version of the Old Testament for the Greek Orthodox Church, but not for the Western Church, which since Jerome, has adhered to the Masoretic text. In the original Greek New Testament, when Jesus quotes the Old Testament, he is made to quote the LXX, which tends to disagree with the Masoretic text.
an ancient translation of the Hebrew Bible into Greek