What is the X in X-mas?
1540s, from Medieval Latin cranium, from Greek kranion "skull, upper part of the head," related to kara (poetic kras) "head," from PIE root *ker- "horn, head" (see horn (n.)). Strictly, the bones which enclose the brain.
cranium cra·ni·um (krā'nē-əm)
n. pl. cra·ni·ums or cra·ni·a (-nē-ə)
The bones of the head considered as a group; skull.
The bony case enclosing the brain, excluding the bones of the face; braincase.
The part of the skull that encloses the brain.