The structure indicated is the lambdoid suture.
Sutures are junctions between the bones of the skull – they are a type of fibrous joint, bound together by Sharpey’s fibres.
Fibrous joints are fixed, immobile joints, with no joint cavity, and connected by fibrous connective tissue. There are three types of fibrous joint.
1) Suture (joints between skull bones)
2) Syndesmosis (joints between long bones of forearm and leg)
3) Gomphosis (joints between roots of a tooth and socket in jaw)
The lambdoid suture joins the parietal bones with the occipital bone, and is continuous with the occiptomastoid suture.
The name of the suture comes from its shape, which resembles the Greek letter Lambda (λ).