Arrachera

Skirt steak is the US name for a cut of beef steak from the plate. It is long, flat, and prized for its flavor rather than tenderness. It is distinct from hanger steak , also called skirt (UK) or onglet, a generally similar adjacent cut also from the plate. Though it is from a different part of the animal, its general characteristics and uses cause it to be confused with both flank steak, taken from the flank behind the plate, and the flap meat from the bottom sirloin behind the flank and above the rear quarter. Both the inside and outside skirt steak are the trimmed, boneless portion of the diaphragm muscle attached to the 6th through 12th ribs on the underside of the short plate. This steak is covered in a tough membrane that may be removed before cooking. By keeping the membrane it will be more tender but would require the eater to remove it which is why many remove it prior to cooking.

Source: Wikipedia