pi·a (p???, p???)n. The pia mater.pi?al adj.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 (?pa??; ?pi??) n1. (Anatomy) anatomy the innermost of the three membranes that cover the brain and the spinal cord2. (Plants) a herb from East India which has an edible root that produces arrowroot starchCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014