Diaphragm (muscle): The muscle that separates the chest (thoracic) cavity from the abdomen. The diaphragm is the main muscle of respiration. Contraction of the diaphragm muscle expands the lungs during inspiration when one is breathing air in. We rely heavily on the diaphragm for our respiratory function so that when the diaphragm is impaired, it can compromise our breathing. The nerve that controls the diaphragm is the phrenic nerve, which originates at vertebral level C3-C5. During development the diaphragm moves down and drags the phrenic nerve with it.