The fat that runs through the flesh of a meat joint or cut
Marbling is the term used to describe the thin seams of fat that run through the fleshy parts of a joint or cut of meat, rather than the layer of fat around the outside under the skin.
Heavily marbled meat is held by many to be much more tender and tasty than meat with little internal fat, and it's certainly true that it has less of a tendency to dry out when cooking.
Many meats, especially pork, are now sold with little or no marbling, which the big-business food retailers say is in response to consumer demand for leaner meat.
However, increasing numbers of people think that this is a bogus claim, and that the real reason is that as marbling takes time to develop it is difficult to achieve with intensive farming techniques which rely on getting an animal to market weight in as short a time as possible.