Definition of diaphragm in Blount's 1707 Glossographia Anglicana Nova The diaphragm can be an upward curved, c-formed structure of muscle mass and fibrous tissue that separates the thoracic cavity through the abdomen.Dysfunction with the diaphragm can cause disruptions in esophageal motility and lead to gastroesophageal reflux sickness (GERD).Anato