Spondylolysis is a disorder of the spine in which part of the fifth (or, rarely, the fourth) lumbar vertebra in the lower back consists of soft, fibrous tissue instead of normal bone.
As a result, the affected vertebra is weak and vulnerable to damage under stress, which may produce spondylolisthesis (forward slippage of the vertebra over the one below it).
If spondylolisthesis does occur, it may produce symptoms such as pain and stiffness and require treatment.
Otherwise, spondylolysis usually produces no symptoms and treatment is not needed.