There are various interpretations of what a Delmonico steak is. The original Delmonico steak was served at Delmonico's Restaurant in the mid-1800s; since then, changes in the beef industry and nomenclature of different primals and cuts leave the Delmonico steak without a clear identity.
In 1973, the National Live Stock and Meat Board simplified what was thousands of names for beef cuts into a list of 300 standard names to be used. Even with this standardization, the Delmonico remains unstandardized. According to Chef's Resources, an informational resource for professional chefs, at least nine different cuts are referred to as Delmonico. The various cuts come from the chuck, rib and sirloin of the cow. Some are boneless while others are bone-in. The Delmonico cut varies by region, butcher and restaurant.
The current Delmonico Restaurant in New York's Delmonico steak is a boneless rib-eye cut from any part of the rib-eye. At Emeril Lagasse's Delmonico Restaurant locations, the Delmonico steak is a bone-in rib-eye steak.
According to Jackson Frozen Food Locker, a meat processing company, in southeast Missouri a Delmonico steak is a bone-in top loin steak, but in other parts of the country a Delmonico steak is a bone-in top loin steak cut from the short loin or a rib-eye steak cut from the rib.