In 1959, the American Association on Mental Retardation (then called the American Association on Mental  Deficiency) adopted this definition of mental retardation:

Mental retardation refers to sub-average general intellectual functioning which originates during the developmental period and is associated with impairment in adaptive behavior." (Heber, 1959, cited in J.D. Smith 381)

The AAMD Manual on Terminology and Classification in 1959  "suggested a three-level division of those with an IQ below 50 into moderately, severely, and profoundly retarded. This classification replaced a similar hierarchy from 1910 based on the "mental age" of the individual – idiots, imbeciles, and morons.

1910 Model "Mental Age"
1910 Model "Mental Age"

In 1961, the AAMD was more specific about what it meant by "sub-average". Anyone scoring below 85 on an IQ test would be considered sub-average. This created a very large category of people defined as "borderline". The generic terms of borderline (IQ 67-83), mild (IQ 50-66), moderate (IQ 3-49), severe (16-32), and profound (IQ <16) were adopted.