What is the definition or description of: slipped disc?

Archaic: Not really a good term, as reflects "something off with a disc". Better is disc bulge, protrusion, herniation, all which can measure aberrant disc positioning in the spine, and can guide therapeutic interventions.

Slipped disc: The disks are protective shock-absorbing pads between the bones of the spine (vertebrae). The disks of the spine are also referred to as intervertebral disks. Although they do not actually "slip, " a disk may move, split, or rupture. This can cause the disk cartilage and nearby tissue to fail (herniate), allowing the inner gel portion of the disk to escape into the surrounding tissue.