Vertebral defect. It is essentially a hairline fracture in the vertebra at the pars. Sometimes this is an incidental finding and does not cause any particular pain or instability. It can be congenital, something you are both with, or traumatic. In some cases, it can lead to pain or instability. A dynamic x-ray, one taken while bending forward and back, can indicate whether this is unstable or not.
A defect in vertebra. This refers to a defect in the bone that connects the upper and lower facet joint of a vertebra. It is thought that most these arise as stress fractures that do not heal. The vast majority cause no symptoms.