The Collaborative International Dictionary of English v.0.48:

Septuagint \Sep"tu*a*gint\, n. [From L. septuaginta seventy.]
   A Greek version of the Old Testament; -- so called because it
   was believed to be the work of seventy (or rather of
   seventy-two) translators.
   [1913 Webster]

   Note: The causes which produced it [the Septuagint], the
         number and names of the translators, the times at which
         different portions were translated, are all uncertain.
         The only point in which all agree is that Alexandria
         was the birthplace of the version. On one other point
         there is a near agreement, namely, as to time, that the
         version was made, or at least commenced, in the time of
         the early Ptolemies, in the first half of the third
         century b.c. --Dr. W. Smith (Bib. Dict.)
         [1913 Webster]

   Septuagint chronology, the chronology founded upon the
      dates of the Septuagint, which makes 1500 years more from
      the creation to Abraham than the Hebrew Bible.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Septuagint
    n 1: the oldest Greek version of the Old Testament; said to have
         been translated from the Hebrew by Jewish scholars at the
         request of Ptolemy II

Easton's 1897 Bible Dictionary:

Septuagint
   See VERSIONS.