Specifically, those who have an IQ between 0 and 25 are idiots; IQs between 26 and 50 are considered imbeciles; and those who have an IQ between 51 and 70 are considered morons.
These terms were popular in psychology as associated with intelligence on an IQ test until around the 1960s.
Source: THE WORDS MORON, IMBECILE, AND IDIOT MEAN DIFFERENT THINGS
In my opinion all the aforementioned IQ ranges are still far above what is, at times, displayed in the US.