The Collaborative International Dictionary of English v.0.48:

Pentateuch \Pen"ta*teuch\, n. [L. pentateuchus, Gr. ?; ? (see
   Penta-) + ? a tool, implement, a book, akin to ? to
   prepare, make ready, and perh. to E. text. See Five, and
   Text.]
   The first five books of the Old Testament, collectively; --
   called also the Law of Moses, Book of the Law of Moses,
   etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Pentateuch
    n 1: the first of three divisions of the Hebrew Scriptures
         comprising the first five books of the Hebrew Bible
         considered as a unit [syn: Torah, Pentateuch, Laws]