The word "Septuagint" is quite slippery. In their first chapter Jobes and Silva note a few different ways the word is used and provide a little etymology: Etymology:
- "Septuagint" came into English from the Latin word Septuaginta ("seventy"), a shortened form of the title Interpretatio septuaginta virorum ("The Translation of the Seventy Men").
- The Latin title arose from the Greek phrase οἱ ἑβδομηκοντα ("the seventy")
- Therefore we refer to the Septuagint with the roman numerals LXX as shorthand.
- In its most general sense "Septuagint" refers to any or all Greek translations of the Hebrew Bible. Something like the way we use "English Bible" without reference to a particular translation. E.g. > When doing Old Testament exegesis the Septuagint is relevant.
- More narrowly it can refer to a particular printed edition of the Greek Old Testament. E.g. > "Get your NA27, your BHS and your Septuagint and meet me at Starbucks." With this sentence I would be referring either to the edition edited by Rahlf and Hanhard or one of the Göttingen volumes.
- Even more narrowly it can be used by scholars to distinguish the oldest translation of the Pentateuch from (1) other later translations of biblical books or (2) revisions of the Greek text. -- A little explanation -- The Pentateuch was translated into Greek sometime during the middle of the third century B.C.E. This is where it all started. Throughout the next couple centuries the rest of the books of the Hebrew Bible were translated and all these translations were revised. Some reserve "Septuagint" for the the original translation of the Pentateuch and refer to the original translation of the other books as "Old Greek," referring the the whole Greek Old Testament with the abbreviation "LXX/OG." The revisions of all these translations are called "recensions." E.g. > "In your paper you should compare and contrast the translation techniques evident in the text of the Septuagint with the translation techniques evident in the Greek text of Proverbs."
Therefore when you see the word "Septuagint," you have to pay attention to the context in which it is being used. On this site it is safe to assume, unless explicitly stated otherwise, that "Septuagint" is being used in the general sense (the first bullet under "usage" above).