noun
: any of three annular segments into which the bones of the cranium proper may be grouped:
a. : an occipital segment consisting of the basioccipital, exoccipital, and supraoccipital bones
b. : a parietal segment consisting of the basisphenoid, alisphenoid, and parietal bones
c. : a frontal segment consisting of the presphenoid, orbitosphenoid, and frontal bones