The Septuagint generally refers to the books of the Old Testament in their old Greek translation. Some scholars only use the term to refer to the Pentateuch section, although mostly the term is used to represent the Greek translation of the entire Old Testament. The correct pronunciation of the term, and the one used by Septuagint scholars, is SEP-tu-jent, the ultimate and penultimate syllables almost sounding as one. Most other folks, however, continue to pronounce the term sep-TU-a-jint.
The origin of the Septuagint is unclear due to fictitious stories surrounding its inception. The most popular is from the letter of Aristeas which describes King Ptolemy Philadelphus’ desire to build a library containing all the books in the world. He requested a copy of the “laws of the Jews” (the OT), and so word was sent to Jerusalem to send 72 elders (6 from each tribe) to work on the translation from the Hebrew to the Greek. Legend states that the Septuagint was written in Ptolemaic Egypt (probably true) from the ancient Hebrew manuscripts of the Old Testament (also probably true). There were 70 (sometimes 72) Jewish elders who independently, and without consultation one with another, each produced their own translation of the entire OT (probably not true). Once completed, the texts were compared with each other, and miraculously none of the translations deviated from each other, but were all in complete and total accordance (total bogus). Various other stories regarding the translation exist, and the reader is encouraged to seek secondary literature on the subject for more. This Greek translation of the OT often goes by the abbreviation “LXX,” the Roman numeral for “70.”
Various mss of the LXX have been found and used, although none of the autographs survive (the usual case with ancient scripture). For the most part, modern go-to sources for the LXX are an eclectic version, much like our current NA27 or UBS4 versions of the NT. Several of these scripts appear in cursive or uncial formats and some vary in their word choices (despite the legends). The Greek of the LXX indicates that it may have been written as early as the 3rd century BC, but scholarly consensus has not been reached. Other postulated dates for the mss of the LXX are mostly later than this, and very, very few hypothesize an earlier date.
Of all the versions and recensions of the LXX, perhaps the most important is Origen’s Hexapla. The Hexapla was a six-column work created by Origen, an Egyptian Christian ca. 240 AD. The various columns of the Hexapla included 1) The Hebrew OT, 2) A Greek transliteration of the Hebrew*, 3) Aquila’s ms, 4) Symmachus’ ms, 5) the LXX, and 6) Theodotion’s ms. Origen wrote the Hexapla in effort to assist Christians in their discussions with Jews, who often used the Hebrew ms of the Bible (Egyptian Jews most likely used the LXX).
One of the main issues surrounding the use for the LXX today is to illustrate that the Hebrew text from which the LXX was translated is, in some places, non-existent today. For example, some parts of the LXX are much longer than they now appear in our MT (cf. Deut. 32:43ff), others are shorter (Josh. 20:4-6 is missing in LXX), and in some cases, like certain pieces of the book of Jeremiah, the text has been heavily re-arranged. Likewise, the order of the books in the XII prophets is re-arranged in the LXX. Many of these problems persisted and puzzled scholars for centuries. With the discovery and publication of the Qumran manuscripts, many of the oddities between the MT and the LXX were explained. At times the deviant portions of the LXX were confirmed by the Qumran versions, indicating that the MT would be the anomaly. The LXX, for these reasons and many others, continues to assist text-critics of the MT (and LXX scholars) in arriving at the most ancient versions of the OT (one of the primary goals of text-criticism).
The LXX was the primary scripture for the early Christian church. Many of the OT quotations in the NT come to us through the LXX, assuming the author did not make his/her own translation of the Hebrew/Aramaic. Because of this, several hermeneutical concessions and interpolations were made from the Greek of the LXX which distanced the Jews from the Christians. One such instance is the LXX version of Isaiah 7:14, which the early church used to proof-text Jesus’ virgin birth. The Hebrew text could read, however, not “virgin,” but ‘almah, or “young woman.” That virginity was inherent in the Greek term is without question, but the Jews felt this was not the case in the Hebrew word ‘almah. Because of the Christian use of the LXX, it was only a matter of time before the Jews began to disband their use of the LXX in favor of the older Hebrew Bible.
For Mormon studies, 2 Nephi 12:16 was the center of attention for quite some time. The BofM version states “upon all the ships of the sea, and upon all the ships of Tarshish.” The Hebrew version does not mention “ships of the sea.” The LXX does not mention “ships of Tarshish,” and in the BofM we have both, as stated here. Several scholars in the church (and outside it!) have done the grunt-work on the anomaly. Other issues are also being used and discovered.