The stomach of a cow, pig, sheep or ox – ox having arguably the best flavour. Tripe is usually sold specially prepared or cleaned for cooking and is very much an acquired taste. Depending on which animal it comes from it will have a different appearance (‘honeycomb’ tripe is reckoned by some to be the best for cooking) but the flavour is the same. A popular way of cooking it in the UK is with onions.
Tripe is often cleaned and blanched at the start of cooking to remove any microbes or strong aromas. This involves placing the tripe in a large pan of cold water which is slowly heated to the boil, so that the impurities rise to the surface. The butcher may have done this for you, so ask.
Because tripe contains a large amount of connective tissue, it should be slow-cooked to achieve a tender result.