Sciatica refers to pain that travels along the path of the sciatic nerve. The sciatic nerve travels from the lower back through the hips and buttocks and down each leg. Sciatica most often occurs when a herniated disk or an overgrowth of bone puts pressure on part of the nerve. Sciatica is the name given to pain caused by irritation of the sciatic nerve. Anything that irritates this nerve can cause pain, ranging from mild to severe. Sciatica is usually caused by a compressed nerve in the lower spine.