chateaubriand is the name given to a large piece of fillet steak, either much thicker than usual or big enough to serve at least two people, or both. There is some disagreement, e.g. between French and American butchers, over the exact size and nature of the cut.
A tedious accretion of tales about the origin of the name was robustly hacked out of the way by Dallas (1877) in Kettner’s Book of the Table; indeed, the author of this would have gone further and banished the term altogether, as had the members of a certain London club (so he tells us) when a fancy chef sought to install it in their menu.
