The Collaborative International Dictionary of English v.0.48:
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. [1913 Webster]WordNet (r) 3.0 (2006):
pia mater n 1: the highly vascular innermost of the 3 meninges