It is called the Septuagint because legend has it that seventy Hebrew scholars each translated the Scriptures into Greek only to find that they all had created the exact same translation. Thus, it is the translation of the seventy - septuaginta or LXX.
What is the place of the Septuagint in the Catholic Church? Well, it is our official version of the Old Testament. I think that it was Matt Leonard of the St Paul Biblical Institute who pointed out to me that the Vatican II document Dei Verbum on Sacred Scripture states:
"The Church from the very beginning accepted as her own that very ancient Greek translation; of the Old Testament which is called the Septuagint; and she has always given a place of honor to other Eastern translations and Latin ones especially the Latin translation known as the Vulgate" (Dei Verbum, 22).Note that the Septuagint was "her own" from "the very beginning". Not even the Vulgate gets that much attention. The Latin Vulgata and the other Eastern translation, e.g Syriac, hold a place of honor but are not accorded the status of the Greek Septuagint. It should be noted that the Latin Vulgate follows the Old Testament of the Greek Septuagint. Why is this? Because the authors of Scripture most often quote from the Septuagint and not the Hebrew Scriptures. In fact, there are often times when the Apostles agree with the Septuagint against what we know as the Hebrew Masoretic text. More on this later.