Spondylolisthesis means a vertebra in the spine has either slipped forward or backward onto the bone directly beneath it. While it most often occurs in the lower back, it can happen anywhere in the spine. 

If the vertebra has slipped backward due to severe arthritis, it’s a condition called retrolisthesis. If a vertebra has moved forward because of a congenital disability or traumatic injury, it’s a condition known as forward anterolisthesis.