Source : Webster's Revised Unabridged Dictionary (1913)

Craniotomy \Cra`ni*ot"o*my\ (kr?`n?-?t"?-m?), n. [Cranium + Gr.
   ???? to cut off.] (Med.)
   The operation of opening the fetal head, in order to effect
   delivery.

Source : WordNet®

craniotomy
     n : a surgical opening through the skull