Cerebrospinal Fluid (CSF) leak: It is an abnormal flow of CSF from an opening or tear in the meninges (brain covering including the arachnoid and dura) that surround the brain. This dangerous situation can occur after surgery for a brain tumor, pituitary tumor or spinal tumor or can be caused by a head injury and skull fracture.  Prompt repair of CSF leaks is indicated to prevent meningitis (CSF infection) and other potential complications. Learn more.