Spondylolysis and Spondylolisthesis
Spondylolysis is a defect in the pars interarticularis, the portion near the joints between two vetebrae bones in the back.
Spondylolisthesis is the term to describe the forward slipping of a bone (vetebrae) over another beneath it in the spine.
Both these conditions most commonly occur in the lower back and hence are often found in people who suffer from lower back pain. Spondylolysis can occur on its own, and can cause the bones to slip forward causing spondylolisthesis.
Hu, S. S., C. B. Tribus, et al. (2008). "Spondylolisthesis and Spondylolysis." The Journal of Bone & Joint Surgery 90(3): 656-671.