I've seen this word thrown around all my life. I knew it was a significant Biblical document, somehow different from most original-language Bible texts, but I didn't know the significance.
This morning in my study Bible (ESV Reformation Study Bible) I was reading about the period in between the last prophet in the Old Testament and when Jesus was born. Apparently that's the period when the Septuagint was written.
Now what exactly is it? It's a Greek translation of the Hebrew, first five books of the Bible—the Pentateuch—and several other books after that. I'm not sure how many though.
Jewish intellilects in Egypt wanted to share the Hebrew history with the Gentiles, so they worked to translate the Pentateuch. The following books were translated later. The translation is called the Septuagint (from the Latin septuaginta, meaning "seventy") because it is said that seventy scholars worked to translate it.
The Septuagint is also the translation quoted by the apostles in the New Testament (which originally written in Greek) most of the time.