Wikipedia Definition of Sciatica

Sciatica is generally caused by the compression oflumbar nerves L4 or L5 orsacral nerves S1, S2 or S3, or far less commonly, by compression of thesciatic nerve itself. When sciatica is caused by compression of adorsal nerve root (radix ) it is considered a lumbar radiculopathy (or radiculitis when accompanied with an inflammatory response) from aspinal disc herniation (a herniatedintervertebral disc in thespine ), or from roughening, enlarging, and/or misaligning of thevertebrae  (spondylolisthesis), ordegenerated discs. Sciatica due to compression of a nerve root is one of the most common forms ofradiculopathy .

"Pseudo-sciatica," which causes symptoms similar to spinal nerve root compression, is caused by the compression of peripheral sections of the nerve, usually from soft tissue tension in thepiriformis   or relatedmuscles   (see piriformis syndrome and see below).