A codex (from the Latin caudex for "trunk of a tree" or block of wood, book; plural codices) is a book constructed of a number of sheets of paper, vellum, papyrus, or similar materials, with hand-written content. The book is usually bound by stacking the pages and fixing one edge, and using a cover thicker than the sheets. Some codices are continuously folded like a concertina. The alternative to paged codex format for a long document is the continuous scroll. Examples of folded codices include the Maya codices. Sometimes people use the term for a book-style format, including modern printed books but excluding folded books.

A codex is a book bound in the modern manner, by joining pages, as opposed to a rolled scroll.