I have a degree in Psychology we used the term in our diagnostic manual.
You should get a newer diagnostic manual. The DSM V has stricken "mental retardation" from axis I and II diagnoses in favor of "intellectual disability disorder." (I believe the same was already done on the latest DSM IV TR, but do not have a copy handy to confirm.) This is in response to a federal mandate called Rosa's Law.
In psychology and human services, the title QMRP has been replaced with QIDP.
I think we had DSM IV when I was in college. But it has been a long time. So terms like "Moron", "Idiot" and "Imbecile" aren't used to identify levels of retardation anymore? It is strange that terms that were used to classify people scientifically get changed. I guess once a stigma gets put on a word they need to change them for some reason.
When I was fostering children we had one kid (11 year old) who was classified as retarded as his IQ was calculated at 75. After he was with a year his IQ jumped to 110 and he was not only no longer retarded his IQ was above average. You would think that IQ should be a steady thing and not change that dramatically in one year. They also had labeled him with Aspergers at one point which I don't believe even exists.
Anyway not trying to offend anyone. The constant name changing stuff is of interest to me. "intellectual disability disorder." I guess is fine. It is better than something like "African American" that makes no sense to me. Or "little person" which if I were one I would find offensive.