A sagittal x-ray of a female patient with L4 spondylolysis. Photograph: Spineuniverse.com
"Spondylolysis" - A disorder of the spine in which the arch of the 5th (or, rarely, the 4th) lumbar vertebra consists of soft fibrous tissue instead of normal bone. As a result, the arch is weak and prone to damage, which may produce spondylolisthesis (the slipping forward of a vertebra over the one below it.) Otherwise, spondylolysis is usually symptomless.
"spondylolysis". British Medical Association (2007). Illustrated Medical Dictionary. 2nd ed. London: Dorling Kindersley (DK). p522.