Competitive Commander, usually shortened to cEDH, is Commander played with decks optimized to win as efficiently and consistently as possible. It uses the normal Commander framework but emphasizes fast mana, efficient interaction, tutors, and compact, consistent win conditions.
The easy-to-assemble game-winning combination of Thassa's Oracle and Demonic Consultation is a cEDH staple, while free interaction such as Force of Will or Force of Negation help players stop opposing combos without needing to leave mana open.