The term Delmonico steak can mean a lot of things to a lot of people, and there is little agreement about precisely what steak it correctly refers to. In fact, about the only thing people seem to agree on is that it's named for Delmonico's, a steak house in New York where it's said to originate.
[Also see: What is the Best Steak?]
Unfortunately, no one can say for sure exactly what cut of meat Delmonico's was serving when it created the Delmonico steak, because the original restaurant opened in 1837, and closed in 1917, so there's no one around who remembers.
As such, neither butchers nor chefs nor steak aficionados can even agree on whether a Delmonico steak should be boneless or bone-in. And there's even less agreement about exactly which cut of beef it should be made from.
This uncertainty has led to a kind of free-for-all, in which anyone with an apron and a pair of tongs can claim that their steak is the authentic Delmonico. And who's going to stop them? There are no culinary police.
Some say a Delmonico is a smaller version of a T-bone steak. Others insist that it's a bone-in (or boneless) ribeye steak, or perhaps a strip steak—again, could be a boneless one, or it could be bone-in.
Delmonico Steaks Are BIG
At its most basic, though, a Delmonico steak is a big steak—possibly up to two inches thick. And it should be a very high-quality piece of meat, with plenty of marbling.
After all, if you're going to eat a large steak, it should be a good one.
Delmonico Steaks Are Tender
So, we have a mystery. We do know that the Delmonico is a tender cut of meat, which should be cooked quickly with dry-heat cooking methods such as grilling and broiling.
(Also see: How to Cook a Steak.)
As it happens, it's an increasingly common practice among butchers to take chuck eye steaks, tie them up with string and call them Delmonico steaks. Which is about the same as taking a Chevy, sending it through the car wash and calling it a Cadillac.
The reason butchers can get away with this in the first place is because of the fact that there is no strict definition of the word "Delmonico." Still, using a semantic loophole to take advantage of unsuspecting customers is not something a good butcher ought to do.
Ultimately, the Delmonico steak is as much myth as it is a specific cut of steak. Basically, if it's a thick, good quality steak from the rib or short loin, you could call it a Delmonico.
For what it's worth, there is still a Delmonico's Restaurant in New York City (albeit with different owners and in a different location), and on their menu, the Delmonico steak is a boneless ribeye.