A Warhammer 40K codex is a book that explains the units and models each army in the Warhammer 40,000 tabletop game can use when playing a game.
The use of the name is based on the Codex Astartes. This tome was written by Roboute Guilliman, the Ultramarines Primarch, giving deep details on how a Space Marine Chapter should be organized.
Codices (the plural for codex) continue the same edition publication history as the Warhammer 40,000 tabletop game itself.