adjective. 1. with regard to the nervous system, found or taking place somewhere exterior to the brain and spinal cord. 2. with regard to vision, toward the margins of the visual region, instead of near to the center. The beginning of a stimulant generally draws attention to that location. 3. positioned on the exterior region of a body. 4. positioned away from a center and close to the exterior edge. 5. superficial or incidental.