Intellectual disability, formerly known as mental retardation, is characterized by deficits in cognitive functioning and adaptive behavior, affecting a child's ability to learn and develop compared to their peers. It is classified based on severity levels determined by IQ scores, ranging from mild to profound, and affects approximately 6.5 million people in the United States. Causes of intellectual disability include prenatal factors like chromosomal disorders and maternal malnutrition, perinatal factors like low birth weight, and postnatal issues such as child abuse and environmental influences.