The Collaborative International Dictionary of English v.0.48:

Octapla \Oc"ta*pla\, prop. n.; etymol. pl., but syntactically
   sing. [NL., fr. Gr. 'okta- (for 'oktw` eight) + -pla, as in
   E. hexapla; cf. Gr. ? eightfold.]
   A portion of the Old Testament prepared by Origen in the 3d
   century, containing the Hebrew text and seven Greek versions
   of it, arranged in eight parallel columns.
   [1913 Webster]