‘Codex’ is a descriptive word which refers to an ancient book, in the form of books as we know books in our day and age. In the ancient world, the Latin word ‘codex’ meant “tree trunk” and while tablets were used to record events and various documents before Christ, the record of writings from the Roman era, tablets made of wood were used.
In later centuries Christian texts began to be formed as ‘Codex’ of which the pages of early Christian books were made of papyrus.
Many early Christian documents were written on animal skin, which is known as a ‘parchment’ and when formed in a book, is called a ‘Codex’. Many of these books were written or transcribed by monks or by ministers of various congregations for public reading and personal reflection.
So to define the word ‘Codex’ it refers to an ancient book, in the form of a book as we know a book today.