Alpha-1 antitrypsin deficiency: An inherited disorder characterized by a lack of the alpha-1 antitrypsin protease inhibitor. Alpha-1 antitrypsin deficiency leads to damage of various organs, especially the lung and liver. Symptoms may become apparent at a very early age or anytime later, manifesting as shortness of breath due to emphysema or as liver symptoms such as jaundice, fatigue, fluid in the abdomen, mental changes, or gastrointestinal bleeding. Treatment options include, for lung disease, replacement of the missing alpha-1 antitrypsin. Avoidance of smoking and of other lung irritants is an important part of management. Treatment of the liver disease is liver transplant. Also known as protease inhibitor 1 deficiency.