Pork belly is the traditional name in United States. Outside that tradition, butchers carry comparable pork cuts under different names — sometimes the same anatomical piece, sometimes a close cousin. The alternatives below are grouped by country so you can match what your local butcher actually carries.
Lamb breast is structurally the lamb equivalent of pork belly — alternating fat and lean layers that reward slow roasting or braising.
Both come from the belly/underside of the pig and share rich fat content and flavour, but belly is boneless while spare ribs are bone-in from the same general area.
Same anatomical cut — pancetta is the salt-cured form of fresh pork belly. American bacon is the smoked-and-cured equivalent.
Adjacent primal — lardo is from the back fat (along the loin), pork belly is from the underside. Both are pork-fat cuts but anatomically distinct.