Caul can denote the large lobe or flap of the liver, the portion of the pericardium or covering around the heart, or, in another passage, an ornamental head covering. The precise bodily reference varies by context.
In sacrificial regulations, the caul is associated with the liver, kidneys, and fat as a specified portion. In Hosea it refers to an enclosure or covering of the heart, possibly the pericardium.
The term derives from the idea of something redundant or enclosed. In Isaiah it describes a netted or ornamental head-dress.