Source : Webster's Revised Unabridged Dictionary (1913)

Inion \In"i*on\, n. [NL., fr. Gr. 'ini`on the back of the head.]
   (Anat.)
   The external occipital protuberance of the skull.

Source : WordNet®

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