People with Down syndrome may have an intellectual disability. However, the Down syndrome community is strongly against using the ‘r-word’ when describing people with disabilities.
From r-word.org: “When they were originally introduced, the terms “mental retardation” or “mentally retarded” were medical terms with a specifically clinical connotation; however, the pejorative forms, “retard” and “retarded” have been used widely in today’s society to degrade and insult people with intellectual disabilities. Additionally, when “retard” and “retarded” are used as synonyms for “dumb” or “stupid” by people without disabilities, it only reinforces painful stereotypes of people with intellectual disabilities being less valued members of humanity.”