slipped disk

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Protrusion of a part of an intervertebral disk through the fibrocartilage, occurring usually in the lower lumbar region and often causing back pain or sciatica.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun pathology A vertebral disk that protrudes into the spinal canal, exerting pressure on spinal nerves and causing backache or sciatica.