Source : Webster's Revised Unabridged Dictionary (1913)

Pia mater \Pi"a ma"ter\ [NL., fr. L. pia (fem. of pius tender,
   kind) + mater mother.] (Anat.)
   The delicate and highly vascular membrane immediately
   investing the brain and spinal cord.

Source : WordNet®

pia mater
     n : the highly vascular innermost of the 3 meninges