Near the middle of the squamous part of occipital bone is the external occipital protuberance, the highest point of which is referred to as the inion.

The inion is the most prominent projection of the protuberance which is located at the posterioinferior (rear lower) part of the human skull.

The nuchal ligament and trapezius muscle attach to it.

The inion (ἰνίον, iníon, Greek for the occipital bone) is used as a landmark in the 10-20 system in electroencephalography (EEG) recording.

[1] This article incorporates text in the public domain from page 185 of the 20th edition of Gray's Anatomy (1918)