[Flap Steak, Ranchera]
This cut is little known in much of the U.S. but is common in California because it's excellent for Fajitas and other Mexican dishes that use beef cut into narrow strips and also for Southeast Asian and Chinese stir fry dishes which also use narrow strips. Because it's already thin it needs less cutting than other cuts. The photo specimen was 10 inches by 5 inches and 3/4 inch at the thickest point, but most of it was thinner. Weight was 13-1/4 ounces.
Flap meat is cut from the flank at the bottom of the sirloin section of the cow. Unlike Flank Steak, which is ultra lean, Flap meat has a loose, grainy, marinade absorbent texture with enough fat and connective tissue to keep it moist, provided it is not overcooked.
More on Cuts of Beef.