Sweetbread is the name of a dish made of the thymus or pancreas of a young animal. It is not a bread.

The organs serve very different biological purposes, but look fairly similar and so are considered, when cooking, to be comparable. Thymus sweetbreads are slightly longer and more irregular, with pancreas sweetbreads being larger and more rounded.

External link

• What are sweetbreads really? (from The Straight Dope)