The caul is a large web of fat which encloses the intestines. It is not the fatty frill called mudgeon or mesentery that actually holds the intestines together—what the French call fraise—but a cloth-like semi-transparent sheet about a metre square, or a little less. If you see it at all in a butcher’s shop, it will most likely be hanging in a greyish-yellowy-white droop, looking like a worn-out dishcloth. Unappetising. Something you would never think of asking for unless you knew its value and usefulness….
Supposing you run a bit of caul fat to ground, you should soak it in warm water (with a splash of vinegar if it looks particularly unappetising or smells slightly odd). Then you will be able to stretch it out slowly, slowly, and its beauty will be revealed.
Now you can appreciate the names it has been given. Caul used to apply mainly to the little netted caps that people wore. Crépine, which is the French word, is related to crêpe meaning both pancake and those kinds of crinkled cloth known as crêpes and crêpe de chine.