mental retardationn. Often Offensive Intellectual disability.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.mental retardation n (Psychiatry) psychiatry the condition of having a low intelligence quotient (below 70) Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014men?tal retarda?tion n. a developmental disorder characterized in varying degrees by a subnormal ability to learn, a substantially low IQ, and impaired social adjustment. [1900?15] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.mental retardation Below-average intellectual ability resulting from genetic defect, brain injury, or disease, and usually present from birth or early infancy.The American Heritage® Student Science Dictionary, Second Edition. Copyright © 2014 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.