This literally means spine slippage, where one of the 33 bones of the vertebra slips forward relative to its neighbour.  It usually occurs towards the base of the spine in the lumbar area and may exist at birth, develop during childhood or be acquired through stress to the spine, typically from carrying heavy weights. Physical sport such as weightlifting, gymnastics, or football can typically cause wear and degeneration in the spine and males are more likely than females to develop symptoms from the disorder. 

Spondylolisthesis is graded 1 to 5 according to degree of severity, with 5 meaning the vertebral body has completely fallen off the body beneath.  Approximately 5-6% of adult males, and 2-3% of females have a spondylolisthesis and it becomes more common with the increased physical activities of adolescence and adulthood, along with the wear- and-tear of daily life.