TOS stands for Thoracic Outlet Syndrome. You can feel the thoracic outlet on each side of your neck. Specifically, the thoracic outlet is the hollow space above your collarbone, between your shoulder and your neck. Slide your hand down either side of your neck to find your thoracic outlets.
Three vital structures pass through the thoracic outlet: an artery, a vein, and a network of nerves. If compression of one or more of these structures causes symptoms, thoracic outlet syndrome is present.