Merriam-Webster Collegiate Dictionary - mental retardation

 noun  Date: 1914 subaverage intellectual ability equivalent to or less than an IQ of 70 that is accompanied by significant deficits in abilities (as in communication or self-care) necessary for independent daily functioning, is present from birth or infancy, and is manifested especially by delayed or abnormal development, by learning difficulties, and by problems in social adjustment  • mentally retarded adjective