Mental retardation (MR) is a generalized disorder appearing before adulthood, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors. It has historically been defined as an Intelligence Quotient score under 70. Once focused almost entirely on cognition, the definition now includes both a component relating to mental functioning and one relating to individuals’ functional skills in their environment. As a result, a person with a below-average intelligence quotient (BAIQ) may not be considered mentally retarded. Syndromic mental retardation is intellectual deficits associated with other medical and behavioral signs and symptoms. Non-syndromic mental retardation refers to intellectual deficits that appear without other abnormalities.

Mental retardation is a subtype of intellectual disability, although that term is now preferred by most advocates in most English-speaking countries as a euphemism for MR. However, intellectual disability is a broader concept and includes intellectual deficits that are too mild to properly qualify as mental retardation, too specific (as in specific learning disability), or acquired later in life, through acquired brain injuries or neurodegenerative diseases like dementia. Intellectual disabilities may appear at any age. Developmental disability is any disability that is due to problems with growth and development. This term encompasses many congenital medical conditions that have no mental or intellectual components, although it, too, is sometimes used as a euphemism for MR.