The thymus glands of veal, beef, and pork. There are two glands, one near the heart (which is considered more delicious) and one near the throat.
Look for white, plump, and firm sweetbreads, and use within 24 hours of purchase. To remove the outer membrane, the glands are normally soaked in salt water before being cooked.
They can be prepared a variety of ways and are sometimes used to make pate.