What are the differences between retardation, developmental delay and mental impairment?

Good question: Retardation indicates a low IQ and expected problems meeting milestones. Mentally retarded individuals do have developmental delay. Developmental delay can exist alone and means one is not meeting milestones on time. Pt, ot, speech are therapies than can improve developmental delay. Mental impairment is a lot of different diagnosis...Bipolar, dementia, retardation, schizophrenia, etc

Words matter: The definition and practical use of terms varies over time and sometimes with changing attitudes. Retardation is a term that comes from the root verb meaning to "retard" or slow. For many years the adjective retarded was a descriptor used for anyone with developmental delay or impairment. Developmental delay implies not meeting expected progress. Mental impairment can be lifelong or acquired.