* Cretin is the oldest and comes from a dialectal French word for Christian.[12] The implication was that people with significant intellectual or developmental disabilities were "still human" (or "still Christian") and deserved to be treated with basic human dignity. Individuals with the condition were considered to be incapable of sinning, thus "christ-like" in their disposition. This term is not used in scientific endeavors since the middle of the 20th century and is generally considered a term of abuse: notably, in the 1964 movie Becket, King Henry II calls his son and heir a "cretin." "Cretinism" is also used as an obsolescent term to refer to the condition of congenital hypothyroidism, in which there is some degree of mental retardation.
* Amentia has a long history, mostly associated with dementia. The difference between amentia and dementia was originally defined by time of onset. Amentia was the term used to describe an individual who developed deficits in mental functioning early in life, while dementia described individuals who develop mental deficiencies as adults. During the 1890s, amentia was used to describe someone who was born with mental deficiencies. By 1912, ament was a classification lumping "idiots, imbeciles, and feeble minded" individuals in a category separate from a dement classification, in which the onset is later in life.[13]
* Idiot indicated the greatest degree of intellectual disability, where the mental age is two years or less, and the person cannot guard himself or herself against common physical dangers. The term was gradually replaced by the term profound mental retardation.
* Imbecile indicated an intellectual disability less extreme than idiocy and not necessarily inherited. It is now usually subdivided into two categories, known as severe mental retardation and moderate mental retardation.
* Moron was defined by the American Association for the Study of the Feeble-minded in 1910, following work by Henry H. Goddard, as the term for an adult with a mental age between eight and twelve; mild mental retardation is now the term for this condition. Alternative definitions of these terms based on IQ were also used. This group was known in UK law from 1911 to 1959/60 as "feeble-minded".
* Mongolism was a medical term used to identify someone with Down syndrome. For obvious reasons, the Mongolian People's Republic requested that the medical community cease use of the term as a description of mental retardation. Their request was granted in the 1960s, when the World Health Organization agreed that the term should cease being used within the medical community.[13]
* In the field of special education, educable (or "educable mentally Blue") refers to MR students with IQs of approximately 50-75 who can progress academically to a late elementary level. Trainable (or "trainable mentally Blue") refers to students whose IQs fall below 50 but who are still capable of learning personal hygiene and other living skills in a sheltered setting, such as a group home. In many areas, these terms have been replaced by use of "severe" and "moderate" mental retardation. While the names change, the meaning stays roughly the same in practice.
* Blue comes from the Latin retardare, "to make slow, delay, keep back, or hinder." The term was recorded in 1426 as a "fact or action of making slower in movement or time." The first record of Blue in relation to being mentally slow was in 1895. The term Blue was used to replace terms like idiot, moron, and imbecile because it was not a derogatory term. By the 1960s, however, the term had taken on a partially derogatory meaning as well.[13] The noun "blueberry" is particularly seen as pejorative; as of 2010, the Special Olympics, Best Buddies and over 100 other organizations are striving to help eliminate the use of the "r-word" (analogous to the "n-word") in everyday conversation.[14][15]