Asthma: A common lung disorder in which inflammation causes the bronchi to swell and narrow the airways, creating breathing difficulties that may range from mild to life-threatening. Symptoms include shortness of breath, cough, wheezing, and chest tightness. The diagnosis of asthma is based on evidence of wheezing and is confirmed with breathing tests. Many allergens and irritants can precipitate attacks of asthma. Avoidance of precipitating factors can be helpful. Treatment may include lifestyle changes, activity reduction, allergy shots, and medications to prevent or reverse the bronchospasm.