mental disordern. Any of various disorders, such as schizophrenia, bipolar disorder, or autism spectrum disorder, characterized by a distressing or disabling impairment of an individual’s cognitive, emotional, or social functioning.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 disorder n (Law) law (in England, according to the Mental Health Act 1983) mental illness, arrested or incomplete development of mind, psychopathic disorder, or any other disorder or disability of the mind mentally disordered adjCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014