Lactose intolerance: The inability to digest lactose, a component of milk and some other dairy products. The basis for lactose intolerance is the lack of an enzyme called lactase in the small intestine. The most common symptoms of lactose intolerance are diarrhea, bloating, and gas. The diagnosis may be made via a trial of a lactose-free diet or by special testing. Treatment involves avoidance of products that contain lactose or use of lactase enzyme supplements before eating. See also deficiency, lactase.