Hematoma: a collection of blood from abnormal bleeding. Intracranial hematomas may form over the surface of the dura (epidural), under the dura (subdural), under the arachnoid membrane (subarachnoid hemorrhage) or within the brain (intracerebral or intraparenchymal). Hematomas may arise from trauma, bleeding into a tumor, from hypertension, excessive anticoagulation medication or after intracranial surgery. Depending upon the size and location, a hematoma may require evacuation typically through a craniotomy. A chronic subdural hematoma is liquefied and can often be removed through a small hole in the bone and placement of a drainage catheter.