Achalasia: A disease of the esophagus that mainly affects young adults. Abnormal function of nerves and muscles of the esophagus causes difficulty swallowing and sometimes chest pain. Regurgitation of undigested food can occur, as can coughing or breathing problems related to entry of food material into the lungs. The underlying problems are weakness of the lower portion of the esophagus and failure of the lower esophageal sphincter to open and allow passage of food. Diagnosis is made by an X-ray, endoscopy, or esophageal manometry.