I'd like the definition of septuagint, please?

Although the initial translation involved only the five books of Moses, the name Septuagint came to refer to the entire Hebrew Scriptures translated into Greek. The remaining books were translated over the next hundred years or so. Rather than being a coordinated effort, the production of the entire Septuagint was a piecemeal accomplishment. The translators differed in their abilities and knowledge of Hebrew. Most books were translated literally, sometimes to the extreme, while other translations were quite free. A few exist in both long and short versions. By the end of the second century B.C.E., all the books of the Hebrew Scriptures could be read in Greek. Despite inconsistent results, the effect of translating the Hebrew Scriptures into Greek went far beyond what the translators could have expected.

Alexander’s policy of Hellenization had made Greek the international language of the world. Even when his realm was overrun by the Romans, common (or, Koine) Greek remained the language of trade and communication among nations. Whether this resulted from a deliberate effort or was a natural development, the Septuagint version of the Hebrew Scriptures quickly found its way into the homes and hearts of many non-Jews previously unacquainted with God and the Law of the Jews. The results were astonishing.

American Heritage® Dictionary: Description of Septuagint

NOUN: A Greek version of the Hebrew Scriptures that dates from the 3rd century b.c., containing both a translation of the Hebrew and additional and variant material, regarded as the standard form of the Old Testament in the early Christian Church and still canonical in the Eastern Orthodox Church. Septuagintal (-jntl)

Additional references: Columbia Encyclopedia