CODEX (Definition)

The word codex comes from the Latin Codex-ICIS. A codex was a polished wooden tablet for writing, subsequently that tablet was replaced by books with paper pages. And although technically the current books are codex this word is used to refer to books handwritten encompassed between the Preclassic antiquity  to the end of the Middle Ages.
Its etymology comes from the Latin Caldeo (Caudex) meaning "table" which in turn is comes from Caudes-ICIS meaning "tree trunk" which contracted as Codex. That is the name given by the Romans to the writing tablet.

image of the Calixtine Code written in the twelfth century taken from the wikipedia, copyright expired