The Septuagint is the name given to a collection of Greek translations of the books of the Old Testament and the Apocrypha. The earliest translations were done in the 3rd century BCE and the rest over the course of the next two centuries. According to legend, the work of translation was initiated by the Greek King of Egypt, Ptolemy II Philadelphus, who asked 72 scholars to translate the Jewish Torah into Koine Greek so that a copy of their translation might be placed in the Great Library at Alexandria. (Koine Greek was the version of Greek commonly spoken throughout the Eastern Mediterranean at that time - koinos in Greek means 'common'. Koine, which is also the language of the New Testament, is a simpler, less 'literary' version of the langiage than Attic Greek, the dialect in which many of the great authors of the classical period wrote.)
The word Septuagint (pronounced with a soft 'g', as in 'gentle') comes from the Latin word for 'seventy', septuaginta. Seventy is the nearest round number to seventy two - i.e., the number of scholars who were asked by King Ptolemy II to undertake the translation of the Torah .