Spondylolisthesis(spinal bone slipping forward)
This term refers to one spine bone slipping forward on another
(spondy = vertebra, olisthesis = slip)
If you follow a line along the backs of the vertebral bodies, the forward slippage can be better visualized.
One condition that will predispose to these forward slips (spondylolisthesis) is a non healing fracture (spondylolysis) of the back part of the spine, the pars interarticularis, which literally means the part between the joints. This condition, which can often be seen on xrays (as in the xray above at the L5S1 level) can be a source of instability and lead to forward slippage.