An operation in which a piece of the skull is removed. A craniotomy may be done so doctors can remove a brain tumor or abnormal brain tissue. It may also be done to remove blood or blood clots from the brain, relieve pressure in the brain after an injury or stroke, repair a brain aneurysm (a bulge in a blood vessel wall) or skull fractures, or treat other brain conditions. The piece of skull that is removed is usually put back in place after the brain problem has been treated.