While the above term may be considered "politically correct", the more accepted term is actually developmentally disabled. I have worked with individuals with developmental disabilities on and off for several years. If you want to get technical, the actual medical terminology, used by professionals, is mentally retarded. This ranges from mild to profound. This term is not popular among the general public because most people feel like it insults the individual with the disability. IMHO, they are human, and as long as you treat them as such, their title won't really matter to them.