Spinal fusion: A surgical procedure in which two or more of the vertebrae in the spine are united together so that motion no longer occurs between them. Spinal fusion may be done to treat a number of conditions, including scoliosis, deformity, fracture, and disc disease. Bone grafts are usually placed around the section of the spine to be fused. The body then heals the grafts over several months, which joins the vertebrae together. The bone for the graft may be taken from another bone in the patient (autograft) or from a bone bank (allograft). Metal rods, plates, screws, and cages may also be used as an internal splint to hold the vertebrae together while the bone grafts heal and the spine fuses.