the outermost, toughest, and most fibrous of the three membranes covering the brain and spinal cord
See dura mater in American Heritage Dictionary 4
(do͝orˌə māˈtər, mäˈ-, dyo͝orˌə)
noun
The tough fibrous membrane covering the brain and the spinal cord and lining the inner surface of the skull. It is the outermost of the three meninges that surround the brain and spinal cord.