What is the definition or description of: pars defect?

Spondylosis: Spondylolysis is a defect of a vertebra. More specifically it is defined as a defect in the pars interarticularis of the vertebral arch. The great majority of cases occur in the lowest of the lumbar vertebrae (l5), but spondylolysis may also occur in the other lumbar vertebrae, as well as in the thoracic vertebrae. Spondylolysis occurs in three to six percent of the population.