Lamb Loin in the carcass of a lamb comes just before the Hind End of the lamb.
The area can be left as a single joint, either on the bone, or boned and rolled. When rolled, it can be stuffed.
It has a T-Bone running through it. Reasonable marbling makes it a nice joint to roast, but as it’s small, it’s easy to overcook it and make it dry.
If sold whole, with the other side of the animal attached (via the backbone), it’s called a Saddle of Lamb, and used for roasting;
If cut into chops with the T-bone left in, these are Loin Chops;
Cuts from this area can be expensive.