The Collaborative International Dictionary of English v.0.48:

Dura mater \Du"ra ma"ter\ [L., lit., hard mother. The membrane
   was called mater, or mother, because it was formerly thought
   to give rise to every membrane of the body.] (Anat.)
   The tough, fibrous membrane, which lines the cavity of the
   skull and spinal column, and surrounds the brain and spinal
   cord; -- frequently abbreviated to dura.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dura mater
    n 1: the outermost (and toughest) of the 3 meninges [syn: dura
         mater, dura]