Codex

Codex comes from the Latin word for "book." If you want a book to sound really fancy, render its title in faux-Latin and stick the word "Codex" in front, like "Codex: Canus Latinicus."

In Warhammer 40,000

Since the Second Edition of Warhammer 40,000, each army book and expansion published by Games Workshop has been titled "Codex: Insertfactionhere." The term "Codex creep" refers to the fact that the Codices currently in play have been published across three editions and ten years, and it seems that the latest Codices (especially the half-dozen flavors of Space Marines) are more powerful than the older ones in order to entice people to buy the army of the month.

Sometimes the different classes of codices and expansions are nicknamed with some descriptive term ending in "-dex" (cf. the different flavors of Carnifex nicknamed with something ending in "-fex"), like the "mini-dex" books from Third Edition that contained special rules and a few extra units for an army list that has been published already (e.g. Craftworld Eldar as an expansion for Eldar, or Catachans as an expansion for Imperial Guard).

The word Codex is also used in the universe of Warhammer 40,000, like the Codex Astartes.

It should be noted that the Codex release and update schedule is of great concern to the 40k fanbase, and thus all of /tg/. Once the slightest hint of a new codex being released is given to one, sweaty neckbeard, all of the internet knows about it by days end. Once such an inkling is heard, rumors of the new Codex being overpowered, pandering to the latest edition's new features, screwing the owning faction's canon over, laying a plague upon our crops, and draining the light from the sun itself abound with fervor and gusto, dividing the community, and especially the specific faction's playerbase, into a veritable rage-rainbow of skubdivisions.

The especially hot-button issue in the inevitable debate once the shadow of an unannounced new Codex looms is who the writer will be. In the eyes of many players and fans, this and this alone will determine if the Codex will be "bad" or "good", fun and balanced or overpowered and min-maxy. With the rise of Matt Ward (say what you will about him, the man churns out books at a Stephen King-esque rate) and a strange dip in the variety of GW Codex writers, there is now an inevitable list of writers that every Codex will have attached to it, before the real writer is announced.

The Inevitable List will Always Include: Matt Ward, Phil Kelly, Aaron Dembski-Bowden, Dan Abnett, Joss Whedon, Stephenie Meyer, Philip Reeve, Scott Westerfeld, Terry Pratchett, George Lucas, Neil Stephenson, Neil Gaiman, J.K. Rowling, J.R.R Tolkien, M. Night Shamalan, Moira Brown, Bilbo Baggins, Shakespeare, Plato, God, Satan, and Egoraptor.

In Warhammer Fantasy Battle

The army list books and expansions for Warhammer Fantasy Battle are called Army Books for no actual reason, so many 40k players who are also WFB players just call them Codices. Funny how the Sci-Fi setting has the old fashioned term and Fantasy the more modern term.