Achalasia or acalasia is a failure of a ring of muscle (as a sphincter) to relax (completely). It refers most commonly to esophageal achalasia, which is a neuromuscular disorder of the esophagus characterized by the reduced ability to move food down the esophagus (peristalsis). In addition, the inability of the cardia ( also called lower esophageal sphincter)- to relax in response to swallowing (there is increased LES pressure- spasms).