A bulging disc is one that extends beyond its normal position. The normal position of the disc corresponds with the edge of the vertebra to which it is attached. Usually, bulging discs don’t go any further than 3mm out from the edge of the vertebra.
Bulging discs are also called prolapsed discs. They are not to be confused with disc herniations. In a herniation a tear in the annulus (outer covering of the disc) allows the nucleus pulposus to escape. Bulging or prolapsed discs are not necessarily painful.