Intellectual Disability (ID), a term that has officially replaced 'mental retardation', is diagnosed based on criteria from the American Psychiatric Association’s DSM-5. A diagnosis requires deficits in both intellectual functions and adaptive functioning, with an onset during the developmental period. The three core criteria are:

1. Criteria A - Deficits in General Mental Abilities: Confirmed through both clinical assessment and standardized intelligence testing (e.g., reasoning, problem-solving, academic learning).

2. Criteria B - Impairment in Everyday Adaptive Functioning: Deficits that result in a failure to meet sociocultural standards for personal independence and social responsibility across various environments.

3. Criteria C - Early Onset: The intellectual and adaptive deficits must begin during the individual’s developmental period.