Extravasation and subsequent accumulation of blood in the subdural space between the dural and arachnoidal layers of the meninges. This condition primarily occurs over the surface of a cerebral hemisphere (hematoma, subdural, intracranial), but may develop in the posterior fossa and spinal canal (hematoma, subdural, spinal). Acute (hematoma, subdural, acute) and chronic (hematoma, subdural, chronic) forms exist. [MeSH]