Achalasia is a rare disease of the muscle of the esophagus (swallowing tube). The term achalasia means "failure to relax" and refers to the inability of the lower esophageal sphincter (a ring of muscle situated between the lower esophagus and the stomach) to open and let food pass into the stomach. As a result, people with achalasia have difficulty swallowing food. In addition to the failure to relax, achalasia is associated with abnormalities of esophageal peristalsis (usually complete absence of peristalsis), the coordinated muscular activity of the body of the esophagus (which comprises 90% of the esophagus) that transports food from the throat to the stomach.