Severe intellectual disability (ID) is defined as a type of ID characterized by severely sub-average adaptive functioning and intellectual functioning, with an intelligence quotient (IQ) the range of 20-34. The phenotype is Severe intellectual disability (HP_0010864, a Human Phenotype Ontology identifier). Also known as: Early and severe mental retardation, Intellectual disability, severe, Mental retardation, severe, Severe mental retardation.