pia ma·ter (m??t?r, mä?t?r)n. The fine vascular membrane that closely envelops the brain and spinal cord under the arachnoid and the dura mater.[Middle English, from Medieval Latin pia m?ter : Latin pia, tender + Latin m?ter, mother (translation of Arabic ‘umm raq?qa : ‘umm, mother + raq?qa, tender).]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.pia mater (?pa?? ?me?t?) n (Anatomy) the innermost of the three membranes (meninges) that cover the brain and spinal cord. See meninges[C16: from Medieval Latin, literally: pious mother, intended to translate Arabic umm raq?qah tender mother]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014pi?a ma?ter (?pa? ? ?me? t?r, ?pi ?) n. the delicate, fibrous, and highly vascular membrane forming the innermost of the three coverings of the brain and spinal cord. Compare arachnoid (def. 4), dura mater. [1350?1400; Middle English