Princeton's WordNet defines codex as a "leaf-book, an unbound manuscript. . .(as distinguished from a scroll.)" The plural of codex is codices.
The word "codex" comes from the Latin "caudex," meaning a tree stump or block of wood. An early book was called a codex because it resembled a wooden block.
The first codex was created in the 1st century AD and very quickly replaced the papyrus scroll because it was easier to hold.
The early Christians used codices for their religious books to distinguish their works from Jewish scriptures and pagan writings, both of which were written on papyrus scrolls.
Although codex can be used to refer to any book, it is typically only used today to refer to ancient, handwritten books.