Intellectual Disability (Intellectual Developmental Disorder) is characterized by impaired cognitive functioning and difficulty performing everyday tasks, measured against others of similar cultural, social, age, and gender status. There is not a specific age of onset, but it must occur during development.

Assessment - This includes social, conceptual and practical elements, not basing the diagnosis on the IQ score, but including clinical and standardized measures together across three domains (social, conceptual, practical) of functioning. A diagnosis is made if impairment is severe enough to warrant ongoing intervention in one of the three domains. The level of severity is determined based on the individual's adaptive functioning. Specifiers include: mild, moderate, severe, and profound.