Nonalcoholic fatty liver disease (NAFLD) refers to a wide spectrum of liver diseases ranging from the most common, fatty liver (accumulation of fat in the liver, also known as steatosis), to Nonalcoholic steatohepatitis (NASH, fat in the liver causing liver inflammation), to cirrhosis (irreversible, advanced scarring of the liver as a result of chronic inflammation of the liver). All of the stages of Nonalcoholic fatty liver disease are now believed to be due to insulin resistance, a condition closely associated with obesity. In fact, the BMI correlates with the degree of liver damage, that is, the greater the BMI the greater the liver damage.

The term Nonalcoholic is used because liver disease due to alcohol can show the same spectrum of liver disease as nonalcoholic fatty liver disease; however, patients with nonalcoholic fatty liver disease do not consume excessive amounts of alcohol.