Chateaubriand is a beef steak that traditionally consists of a large front cut fillet of tenderloin grilled between two lesser pieces of meat that are discarded after cooking. While the term originally referred to the preparation of the dish, Auguste Escoffier named the specific front cut of the tenderloin the Chateaubriand.
In the gastronomy of the 19th century, the steak for Chateaubriand could be cut from the sirloin, and served with a reduced sauce named Chateaubriand sauce. It was also traditionally served with mushrooms.