Flank steak is a cut from the abdominal section, just below the loin. Not to be confused with it's neighbor, the skirt steak.
A flank steak is a leaner cut of meat with lots of muscle fibers, it is best seared quickly over medium-high heat (400° to 450°F) for 5 minutes on either side on your grill to medium rare and rest to finish. You can cook it directly on the grates to get some beautiful grill marks or it can be cooked and seared in a cast-iron skillet on the grill.