It is a surgical procedure in which the bone flap is removed in order to access the brain.
This procedure is performed in patients suffering from brain injuries or brain trauma.
A simple skull CT was performed on a young male patient of 18 years after leaving a craniotomy to visualize the results.
Then you can see a 3D CT scan, in which the hole of the removed skull portion is clearly seen
https://www.youtube.com/watch?v=MrA1I4VJPF4
Pretty, not?