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]

Easton's 1897 Bible Dictionary:

Pentateuch
   the five-fold volume, consisting of the first five books of the
   Old Testament. This word does not occur in Scripture, nor is it
   certainly known when the roll was thus divided into five
   portions Genesis, Exodus, Leviticus, Numbers, Deuteronomy.
   Probably that was done by the LXX. translators. Some modern
   critics speak of a Hexateuch, introducing the Book of Joshua as
   one of the group. But this book is of an entirely different
   character from the other books, and has a different author. It
   stands by itself as the first of a series of historical books
   beginning with the entrance of the Israelites into Canaan. (See JOSHUA.)

     The books composing the Pentateuch are properly but one book,
   the "Law of Moses," the "Book of the Law of Moses," the "Book of
   Moses," or, as the Jews designate it, the "Torah" or "Law." That
   in its present form it "proceeds from a single author is proved
   by its plan and aim, according to which its whole contents refer
   to the covenant concluded between Jehovah and his people, by the
   instrumentality of Moses, in such a way that everything before
   his time is perceived to be preparatory to this fact, and all
   the rest to be the development of it. Nevertheless, this unity
   has not been stamped upon it as a matter of necessity by the
   latest redactor: it has been there from the beginning, and is
   visible in the first plan and in the whole execution of the
   work.", Keil, Einl. i.d. A. T.

     A certain school of critics have set themselves to reconstruct
   the books of the Old Testament. By a process of "scientific
   study" they have discovered that the so-called historical books
   of the Old Testament are not history at all, but a miscellaneous
   collection of stories, the inventions of many different writers,
   patched together by a variety of editors! As regards the
   Pentateuch, they are not ashamed to attribute fraud, and even
   conspiracy, to its authors, who sought to find acceptance to
   their work which was composed partly in the age of Josiah, and
   partly in that of Ezra and Nehemiah, by giving it out to be the
   work of Moses! This is not the place to enter into the details
   of this controversy. We may say frankly, however, that we have
   no faith in this "higher criticism." It degrades the books of
   the Old Testament below the level of fallible human writings,
   and the arguments on which its speculations are built are
   altogether untenable.

     The evidences in favour of the Mosaic authorship of the
   Pentateuch are conclusive. We may thus state some of them
   briefly:

     (1.) These books profess to have been written by Moses in the
   name of God (Ex. 17:14; 24:3, 4, 7; 32:7-10, 30-34; 34:27; Lev.
   26:46; 27:34; Deut. 31:9, 24, 25).

     (2.) This also is the uniform and persistent testimony of the
   Jews of all sects in all ages and countries (comp. Josh. 8:31,
   32; 1 Kings 2:3; Jer. 7:22; Ezra 6:18; Neh. 8:1; Mal. 4:4; Matt.
   22:24; Acts 15:21).

     (3.) Our Lord plainly taught the Mosaic authorship of these
   books (Matt. 5:17, 18; 19:8; 22:31, 32; 23:2; Mark 10:9; 12:26;
   Luke 16:31; 20:37; 24:26, 27, 44; John 3:14; 5:45, 46, 47; 6:32,
   49; 7:19, 22). In the face of this fact, will any one venture to
   allege either that Christ was ignorant of the composition of the
   Bible, or that, knowing the true state of the case, he yet
   encouraged the people in the delusion they clung to?

     (4.) From the time of Joshua down to the time of Ezra there
   is, in the intermediate historical books, a constant reference
   to the Pentateuch as the "Book of the Law of Moses." This is a
   point of much importance, inasmuch as the critics deny that
   there is any such reference; and hence they deny the historical
   character of the Pentateuch. As regards the Passover, e.g., we
   find it frequently spoken of or alluded to in the historical
   books following the Pentateuch, showing that the "Law of Moses"
   was then certainly known. It was celebrated in the time of
   Joshua (Josh. 5:10, cf. 4:19), Hezekiah (2 Chr. 30), Josiah (2
   Kings 23; 2 Chr. 35), and Zerubbabel (Ezra 6:19-22), and is
   referred to in such passages as 2 Kings 23:22; 2 Chr. 35:18; 1
   Kings 9:25 ("three times in a year"); 2 Chr. 8:13. Similarly we
   might show frequent references to the Feast of Tabernacles and
   other Jewish institutions, although we do not admit that any
   valid argument can be drawn from the silence of Scripture in
   such a case. An examination of the following texts, 1 Kings 2:9;
   2 Kings 14:6; 2 Chr. 23:18; 25:4; 34:14; Ezra 3:2; 7:6; Dan.
   9:11, 13, will also plainly show that the "Law of Moses" was
   known during all these centuries.

     Granting that in the time of Moses there existed certain oral
   traditions or written records and documents which he was
   divinely led to make use of in his history, and that his writing
   was revised by inspired successors, this will fully account for
   certain peculiarities of expression which critics have called
   "anachronisms" and "contradictions," but in no way militates
   against the doctrine that Moses was the original author of the
   whole of the Pentateuch. It is not necessary for us to affirm
   that the whole is an original composition; but we affirm that
   the evidences clearly demonstrate that Moses was the author of
   those books which have come down to us bearing his name. The
   Pentateuch is certainly the basis and necessary preliminary of
   the whole of the Old Testament history and literature. (See DEUTERONOMY.)


Hitchcock's Bible Names Dictionary (late 1800's):

Pentateuch, the five books of Moses