si·ma (s??m?)n. The lower layer of the earth’s outer crust that underlies the sial and is rich in silica, iron, and magnesium.[si(lica) + ma(gnesium).]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.sima (?sa?m?) n1. (Geological Science) the silicon-rich and magnesium-rich rocks of the earth’s oceanic crust, the most abundant individual rock being basalt2. (Geological Science) the earth’s continental lower crust, probably comprised of gabbro rather than basalt[C20: from si(lica) + ma(gnesia)] simatic adjCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014