According to the National Institutes of Health:
- Intellectual disability refers to a group of disorders characterized by a limited mental capacity and difficulty with adaptive behaviors such as managing money, schedules and routines, or social interactions. Intellectual disability originates before the age of 18 and may result from physical causes, such as autism or cerebral palsy, or from nonphysical causes, such as lack of stimulation and adult responsiveness.
- Developmental disability is a severe, long-term disability that can affect cognitive ability, physical functioning, or both. These disabilities appear before age 22 and are likely to be life-long. The term “developmental disability” encompasses intellectual disability but also includes physical disabilities. Some developmental disabilities may be solely physical, such as blindness from birth. Others involve both physical and intellectual disabilities stemming from genetic or other causes, such as Down syndrome and fetal alcohol syndrome.
So, essentially, in the same way that all roses are flowers, but not all flowers are roses, all intellectual disabilities are developmental disabilities but not all developmental disabilities are intellectual. One generally used distinction not included above is that those with intellectual disabilities have an IQ of 70-75 or below, which may or may not be true for those with developmental disabilities. In the end, though, there are many discrepancies in these classifications. The NIH uses Down Syndrome as an example of a developmental disability, whereas many other sources cite it as an intellectual one. The definitions above are a good general guideline, but as with everything concerning the complexities of the human condition, every case is unique and cannot be so easily defined.
National Institutes of Health Fact Sheet