Traditional bacon in the United States is a cut of meat from the belly of a pig that is cured either in a dry rub or in a liquid brine and may or may not be smoked.  You can also get uncured bacon but I just call that fresh pork belly.  However, depending on where you are from the answer may differ.  For example, Canadian bacon comes from the loin of a pig and has more of a ham taste.  To really put some ideas out there, I have made bacon from pork, goat, sheep, deer and beef.  Granted I still used the belly of the animal just so I could get close to the fat/meat ratio that I prefer in bacon.

This is just my interpretation and may not be yours but I can’t go wrong with bacon.