A codex is an ancient book made of stacked, hand-written pages. A historian might study a medieval codex full of beautiful calligraphy and illustrations decorated with gold leaf.
The ancient Romans invented the codex. When the codex first appeared as a way to bind a manuscript, it was a great improvement over previous methods. One of these was the scroll, a long roll of paper, and another was a wax tablet. Codex is a Latin word used to mean "book of laws," although it's literally "tree trunk." The plural of codex is codices.