A rare disorder of the esophagus, the muscular tube that carries food from the throat to the stomach. It is characterized by enlargement of the esophagus, impaired ability to push food down toward the stomach (peristalsis), and failure of the ring-shaped muscle at the bottom of the esophagus, the lower esophageal sphincter (LES), to relax. It is the contraction and relaxation of the sphincter that moves food through the tube.