The external occipital protuberance of the skull.

A projection in the center of the superior nuchal line of the occipital bone; inion may be absent in cases of occipital torus.

the craniometric point that is the most prominent point at the back of the head (at the occipital protuberance)

(Gr. back of the head): A craniometric landmark located at the apex of the external occipital protuberance.

The inion is the most prominent projection of the occipital bone at the lower rear part of the skull. The ligamentum nuchae and trapezius muscle attaches to it.