Posterior listhesis is a "slip" or compression on the vertebrae of the spine. This can be caused by many different factors from illness to degenerative diseases to injury.
a disc that is slipped out of line to the side of the spine
It means sliding of the lumbar vertebral body toward the back. Retro: rearward listhesis: displacement
-listhesis is the medical terminology combining form meaning slipping, as in anterolisthesis or spondylolisthesis.