Intellectual disability (ID), also called intellectual development disorder (IDD) or general learning disability, and formerly known as mental retardation (MR),

MR is a generalized neurodevelopmental disorder characterized by significantly impaired intellectual and adaptive functioning. It is defined by an IQ score below 70 in addition to deficits in two or more adaptive behaviors that affect everyday, general living. 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 environments.

As a result of this focus on the person's abilities in practice, a person with an unusually low IQ may not be considered intellectually disabled. Intellectual disability is subdivided into syndromic intellectual disability, in which intellectual deficits associated with other medical and behavioral signs and symptoms are present, and non-syndromic intellectual disability, in which intellectual deficits appear without other abnormalities. Down syndrome and fragile X syndrome are examples of syndromic intellectual disabilities.

Intellectual disability affects about 2–3% of the general population. 75–90% of the affected people have mild intellectual disability. Non-syndromic or idiopathic cases accounts for 30–50% of cases. About a quarter of cases are caused by a genetic disorder. Cases of unknown cause affect about 95 million people as of 2013.