Ligaments are tough bands of connective tissue (fibres that support other tissues and organs in the body). Ligaments link two bones together at a joint. They strengthen the joint and limit its movement in certain directions. In the spine, the vertebrae are held together by ligaments.
Together with the spinal muscles, the ligaments give the back its strength. In ankylosing spondylitis, the spinal joints and ligaments and the sacroiliac joints (the joints at the base of the spine) become inflamed. Inflammation in the spine can cause pain and stiffness in the neck and back.
Definition from NHS Choices: Ligaments