These are the definitions used by experts in the U.S. and internationally. The Texas Department of Aging and Disability Services follows the standards in the DSM.
Sources:
1. United Nations/ World Health Organization/ International Statistical Classification of Diseases and Related Health Problems/ 10th Revision/ 2007 (WHO)
2. American Psychiatric Association/ Diagnostic and Statistical Manual of Mental Disorders, 4th Edition (DSM-IV)

Definition of Mental retardation:

(WHO): A condition of arrested or incomplete development of the mind, which is especially characterized by impairment of skills manifested during the developmental period, skills which contribute to the overall level of intelligence, i.e. cognitive, language, motor, and social abilities. Retardation can occur with or without any other mental or physical condition.

(DSM): The DSM lists three criteria considered necessary for a diagnoses of developmental disability: 1) an IQ below 70; 2) significant limitations in two or more areas of adaptive behavior (i.e., ability to function at age level in an ordinary environment); 3) evidence that the limitations became apparent in childhood.

Definitions of levels of mental retardation (WHO, DSM)

Mild mental retardation
(WHO) Approximate IQ range of 50 to 69 (in adults, mental age from 9 to under 12 years). Likely to result in some learning difficulties in school. Many adults will be able to work and maintain good social relationships and contribute to society.

(DSM) IQ level 50 – 55 to 70. About 85% of population identified as having developmental disability or delay.

Moderate mental retardation:
(WHO) Approximate IQ range of 35 to 49 (in adults, mental age from 6 to under 9 years). Likely to result in marked developmental delays in childhood but most can learn to develop some degree of independence in self-care and acquire adequate communication and academic skills. Adults will need varying degrees of support to live and work in the community.
(DSM) IQ level 35 – 49to 50-55. About 10% of population identified as having developmental disability or delay.

Severe mental retardation:
(WHO) Approximate IQ range of 20 to 34 (in adults, mental age from 3 to under 6 years). Likely to result in continuous need of support.
(DSM) IQ level 20-25 to 35-40. About 3% to 4% of population identified as having developmental disability or delay.

Profound mental retardation:
(WHO) IQ under 20 (in adults, mental age below 3 years). Results in severe limitation in self-care, continence, communication and mobility
(DSM) IQ level below 20 or 25. About 1% to 2% of population identified as having developmental disability or delay

Note: Many experts in the United States no longer use the term “mental retardation,” and many have replaced it with “developmental disability” or “developmental delay.” The World Health Organization and the American Psychiatric Association continue to use the term “mental retardation”.

Definition of Adaptive Behavior

Adaptive behavior is the collection of conceptual, social, and practical skills that people have learned so they can function in their everyday lives. Significant limitations in adaptive behavior impact a person's daily life and affect the ability to respond to a particular situation or to the environment.

Limitations in adaptive behavior can be determined by using standardized tests that are normed on the general population including people with disabilities and people without disabilities. On these standardized measures, significant limitations in adaptive behavior are operationally defined as performance that is at least 2 standard deviations below the mean of either (a) one of the following three types of adaptive behavior: conceptual, social, or practical, or (b) an overall score on a standardized measure of conceptual, social, and practical skills.

(Source for adaptive behavior information: (American Association on Intellectual and Developmental Disabilities (AAIDD))