The Collaborative International Dictionary of English v.0.48:

Codex \Co"dex\ (k[=o]"d[e^]ks), n.; pl. Codices. [L. See
   Code.]
   1. A book, especially an early form of book with pages
      stitched together, contrasting with the earlier scrolls; a
      manuscript; as, a DaVinci codex.
      [1913 Webster]

   2. A collection or digest of laws; a code(senses 2 or 4).
      [archaic] --Burrill.
      [1913 Webster]

   3. An ancient manuscript of the Sacred Scriptures, or any
      part of them, particularly the New Testament.
      [1913 Webster]

   4. A collection of canons. --Shipley.
      [1913 Webster]

WordNet (r) 3.0 (2006):

codex
    n 1: an official list of chemicals or medicines etc.
    2: an unbound manuscript of some ancient classic (as
       distinguished from a scroll) [syn: codex, leaf-book]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

CODEX. Literally, a volume or roll. It is particularly applied to the volume
of the civil law, collected by the emperor Justinian, from all pleas and
answers of the ancient lawyers, which were in loose scrolls or sheets of
parchment. These he compiled into a book which goes by the name of Codex.