Metabolic dysfunction-associated steatohepatitis (MASH, formerly known as nonalcoholic steatohepatitis or NASH) is a type of fatty liver disease. It often develops due to a metabolic disorder, such as obesity or diabetes, resulting in a toxic buildup of fat in the liver. It is the most severe form of metabolic dysfunction-associated steatotic liver disease (MASLD, formerly known as nonalcoholic fatty liver disease or NAFLD).