Cranial suture is a fibrous joint, more specifically a Synarthroses joint.
Sutures are fibrous joints that occur only between the bones of our skull or cranium and provide slight mobility. In the old age, the cranial suture may turn to bone (ossification).
Since the cranial suture offers only slight mobility it is also called a synarthroses joint (joint that allow small or no movement). These are called fibrous joints because after the fusion of cranial bones, any leftover gaps are filled with tissue fibers. The relative position of these bones keep on changing through out the life of an individual. The very reason for the safety of our brain is the immobility of the bones, which is brought about by the immovable nature o the cranial suture.