What do the letters stand for in warhammer 40k codexs?

In Warhammer 40,000, "Codex" is short for "Codex Astartes," which refers to the comprehensive guide for Space Marine chapters. However, in a broader context, "Codex" generally denotes a rulebook or supplement for a specific faction or army within the game. Each Codex contains lore, background, rules, and units specific to that faction, allowing players to field their armies in accordance with the game's mechanics.