What is the outward protrusion of the skull behind the ear?

The protruding area behind the ear on the outer surface of the skull belongs to the human mastoid process, located in the posterior portion of the temporal bone.

The mastoid process refers to a thick bony prominence situated below and behind the squamous part of the temporal bone, posterior to the external auditory meatus. It has a conical or cone-like shape and can be palpated manually, forming one of the components of the temporal bone. It consists of the mastoid portion of the temporal bone, mastoid air cells, mastoid antrum, and can be involved in mastoiditis. Functionally, it serves as an attachment site for several muscles—for example, the occipitalis and posterior auricular muscles attach externally to the mastoid, while the sternocleidomastoid and splenius capitis muscles attach to its lower and lateral aspects. Additionally, the interior of the mastoid contains numerous air-filled cavities that help reduce the overall weight of the skull.

Medically, common conditions involving the mastoid include acute mastoiditis and chronic mastoiditis, which are often closely associated with middle ear infections (otitis media). Symptoms may include tenderness behind the ear, fever, and swelling behind the ear. If these symptoms occur, early medical evaluation is recommended to prevent delays in diagnosis and treatment.