A caul is a thin, filmy membrane, the remnants of the amniotic sac, that covers or partly covers the newborn mammal immediately after birth. It is also the membrane enclosing the paunch of mammals, particularly as in pork and mutton butchery.

In childbirth it is seen as a shimmery coating of the head and face. The caul is harmless and it is easily removed by the doctor, midwife, or person(s) performing the childbirth. The appearance of a caul over a newborn baby's head is occasional; not all children have one, though they are not especially rare.

In medieval times the appearance of a caul on a newborn baby was seen as a sign of good luck. It was considered an omen that the child was destined for greatness. Gathering the caul onto paper was considered an important tradition of childbirth: the midwife would rub a sheet of paper across the baby's head and face, pressing the material of the caul onto the paper. The caul would then be presented to the mother, to be kept as an heirloom.

Over the course of European history, a popular legend developed suggesting that possession of a baby's caul would give its bearer good luck and protect that person from death by drowning. Cauls were therefore highly prized by sailors. Medieval women often sold their cauls to sailors for large sums of money; a caul was regarded as a valuable talisman.

In butchery the caul is used as offal.