A cowboy steak, aka "tomahawk" is a particular trim of bone in rib steak. It (or they, if you prefer) is/are cut "chop" style, almost but not always "full bone width" (which makes them very thick), with the bone extending well beyond the meat; the end of the bone is frenched.
Many meat terms are legitimate but regional. That is they have different accepted meanings in different parts of the country or perhaps have an accepted meaning only in one. Many meat terms are fanciful and have only whatever meaning the seller gives them. "Delmonico" is a bit of both. In one sense it refers to the particular cut of steak served at Delmonico's in New York City in the Gilded Age. In another, "Delmonico" is a Gilded Age term for quality. Delmonico steaks can be "strip" or rib, depending.
What really sets the standard for naming are the national IMSA codes. However, since you're not going to order your steak in a restaurant by its IMSA number, it's of limited utility for most non-professional CT readers. Still, the more you know about the codes, the more you know about cutting in general.
Two good online resources for meat wisdom are "Ask The Meatman," and if you're already starting with some background, "Bovine Myology." Good but not comprehensive in the sense that they don't cover all of the regional and idiosyncratic names.
The best rule of thumb: If you don't know, ask.
Edited by boar_d_laze - 12/31/11 at 9:19am