Congenital hip dislocation: One of the most common birth defects, characterized by an abnormal formation of the hip joint in which the ball at the top of the thighbone (the head of the femur) is not stable within the socket (acetabulum). The ligaments of the hip joint may also be loose and stretched. The degree of instability at the hip varies. The usual treatment is the use of a device called the Pavlik harness. If the harness is not effective, the hip may be positioned into place under anesthesia (closed reduction) and maintained with a body cast (spica). Also known as infantile hip dislocation, congenital dislocation of the hip (CDH), and developmental dysplasia of the hip (DDH).