The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

craniotomy
    n 1: a surgical opening through the skull