pia mater
noun
pia ma·ter ˈpī-ə-ˌmā-tər 
ˈpē-ə-ˌmä-
ˈpē-ə-ˌmä- : the thin vascular membrane that invests the brain and spinal cord internal to the arachnoid and dura mater
Word History
Etymology
Middle English, from Medieval Latin, from Latin, tender mother
First Known Use
14th century, in the meaning defined above
Medical Definition
pia mater
noun
pia ma·ter -ˈmāt-ər
: the delicate and highly vascular membrane of connective tissue investing the brain and spinal cord, lying internal to the arachnoid and dura mater, dipping down between the convolutions of the brain, and sending an ingrowth into the anterior fissure of the spinal cord
called also pia