It is a lacy membrane of fat, like a piece of cloth, as opposed to other kinds of fat, which are dense and white.
Caul fat from pig is considered better than from lamb. In pigs, the caul fat surrounds their stomachs.
It is used for wrapping things, such as faggots, pâtés, sheftalias, terrines. It doesn’t add any flavour to speak of, but keeps the item being cooked moist as the fat melts away and bastes it. It will completely disappear by the end of cooking.
It is very hard to buy now; you pretty much have to order it from butchers.
Soak in water before use.
Thin strips of unsmoked fatty bacon (streaky bacon in the UK; American-style bacon in North America).
The French word for caul fat, “crepinette”, gave birth to the English term “crespinette”, meaning “hair net”. A caul was also a type of cap that women in Elizabeth times wore on the back on their heads.
It is also used to describe a membrane sometimes covering a newly-born child. Many superstitions around such cauls being lucky used to exist.