It is an individual who differs under two standard deviations from the mean in terms of mental functions, has deficiencies or limitations in conceptual, cognitive, social and practical adaptation skills, these characteristics appear in the developmental period before the age of 18 and need special education and support education services.

Individual with mild intellectual disability: It is an individual who needs special education and support education services at a limited level due to her mild disability in mental functions and conceptual, social and practical adaptation skills.
Individuals with moderate intellectual disability: It is an individual who needs special education and support education services intensively in order to gain basic academic, daily life and work skills due to the limitations in mental functions and conceptual, social and practical adaptation skills.
Individuals with severe intellectual disability: It is defined as an individual who needs lifelong, consistent and intensive special education and support education services in all areas of life, including the teaching of self-care skills, due to their deficiencies in mental functions and conceptual, social and practical adaptation skills.