A very thin, lace-like fat that comes from a variety of animals, including pork, beef, and lamb. It can be wrapped around a lean cut of meat prior to roasting to enhance flavor and keep it moist. It is also used like sausage casing with forcemeats. When cooked the caul fat melts and becomes an incredibly delicate, almost imperceptible, outer layer on whatever it's been cooked.
It can be difficult to find and is not common in supermarketes, check with your local butcher.