Dr. Corwell covered Spondyloysis in July 2010  https://umem.org/res_pearls_referenced.php?p=1134 but if you are like me you might have trouble remembering the differences between the following terms:

• Spondyloysis: A unilateral or bilateral defect in the pars interarticularis portion of a vertebrae.  Typically L5 or L4.
• Spondylosis: is a term referring to degenerative osteoarthritis of the joints between the spinal vertebrae and/or neural foraminae.
• Spondylolisthesis: describes the anterior displacement of a vertebra or the vertebral column in relation to the vertebrae below. Usually due to spondyloysis or a fracture of the pedicles of the vertebrae.  Can occur anywhere along the vertebral column. Most common at the L4 and L5 level.  For example,  a hangman's fracture is a spondylolisthesis of the C1 vertebra being displaced anteriorly relative to the C2 vertebra.
• Spondylitis: is an inflammation of the vertebra. As can be seen with ankylosing spondylitis, Pott’s disease or any infection or arthritic disorder of the spine.

References