1. Either of the longest pair of cranial nerves mainly responsible for parasympathetic control (activities that occur when the body is at rest) over the heart and many other internal organs, including thoracic and abdominal viscera or abdominal cavity below the diaphragm but not including the pelvic tissues: The nervi vagi communicate through thirteen main branches, connecting to four areas in the brain.
2. Etymology: from Latin, vagus, "wandering" + nervus, "nerve".