Achalasia is the failure of a ring of muscle, such as the lower esophageal sphincter (LES), to relax appropriately at the moment that food arrives at the end of its journey down the esophagus. This muscular non-relaxation creates a functional obstruction, interfering with normal passage of food into the stomach. This term is most commonly used in relation to the LES, but may also be used in reference to the upper esophageal sphincter (UES) or even anus.