Intellectual and/or Developmental Disabilities. It is sometimes called Intellectual disabilility, learning disability (in British based dialects), developmental delay or disability, and, in historical context only, MR.
IDD is defined in many ways.
From AAIDD: Intellectual disability is a disability characterized by significant limitations in both intellectual functioning and in adaptive behavior, which covers many everyday social and practical skills. This disability originates before the age of 18.
Psychology Today breaks down the diagnostic criteria based in the DSM-V