Loin is a term used to describe a cut of meat for all meat mammals. Generally, it describes the muscles of the back side of the lower abdomen, between the shoulders and the rear legs. In all mammalian meat animals, the loin is a prized cut because that section of the animal does very little work so it contains little connective tissue and has very tender meat. It does not describe quite the same muscle on all four-legged animals, however, and usually requires some additional terms to make it clear.
The tenderloin is a long, cylindrical pair of muscles that run down the spine, beneath the ribs. The tenderloin is usually considered the finest cut of meat and is either removed entirely for sale as filet mignon (beef), pork tenderloin, or lamb tenderloin, or is included as a part of a luxury cut like porterhouse or lamb chop.
On beef cattle, loin is one of the eight primal cuts. It is located directly behind the ribs. The portion closest to the center of the cow is short loin, from which the New York strip steak, T-bone steak, porterhouse, and strip loin are cut. Just behind the short loin is the sirloin, which is less tender but more flavorful, and from which is cut the sirloin steak, top sirloin, bottom sirloin, tri-tip roast and tri-tip steak. For veal (young cattle), both tenderloins are included in a saddle of veal.
On lamb, "short loin" is not usually used, with the term "loin" used to describe the area behind the ribs, with sirloin behind that, as in cattle. The loin is the source of lamb loin roast, and lamb loin chops, which are usually just called lamb chops. Lamb is also sometimes cut with both loins intact, called saddle of lamb.
For pork, the "loin" is a large, wide muscle, called by a long list of alternate names, including center cut pork loin roast, center cut pork roast, pork center loin roast, pork center cut rib roast, pork loin center cut, pork loin center rib roast, pork loin roast center cut and pork loin rib half. It is well-suited to roasting or grilling.
Pork tenderloin, also called pork filet or pork tender, is, like beef and lamb, a long, tender cylindrical muscle. It is the tenderest and most delicate pork, suitable for grilling, roasting, or poaching.