A necropsy is a surgical examination of a dead body, most commonly a dead animal, in order to learn why the animal died.
A more common word for necropsy is autopsy. Either way, it's the dissection of a corpse performed to learn something about the cause of death or about a particular disease. Frequently the word necropsy is used when the body being examined is not human — in the case of a laboratory animal, for example. Whenever you see the prefix necro, you can be sure the word has something to do with death. In this case, it's combined with opsis, "a sight" in Greek.