From the Arc of the United States Website
Intellectual disability is a below-average cognitive ability with three (3) characteristics:
Intelligence refers to general mental capability and involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly, and learn from experience.
Studies show that somewhere between one (1) percent and three (3) percent of Americans have intellectual disabilities. There are many causes of intellectual disabilities, factors include physical, genetic and/or social.
The most common syndromes associated with intellectual disabilities are autism, Down syndrome, Fragile X syndrome and Fetal Alcohol Spectrum Disorder (FASD).
Common causes occur from genetic conditions (Down syndrome and Fragile X syndrome are examples), problems during pregnancy (a pregnancy of the mother who drinks alcohol while pregnant can result in FASD), problems at the time of birth, health problems such as whooping cough, measles or meningitis and exposure to environmental toxins like lead or mercury.
The impact of having an intellectual disability varies considerably, just as the range of abilities varies considerably among all people. Children may take longer to learn to speak, walk and take care of their personal needs, such as dressing or eating. It may take longer to learn in school.
As adults, some people are able to lead independent lives in the community without paid supports. A small percentage will have serious, lifelong limitations in functioning. However, with early intervention, an appropriate education and supports as an adult, all can lead satisfying lives in the community.
Sometimes intellectual disability is also referred to as developmental disability which is a broader term that includes ASD (autism spectrum disorders), epilepsy, cerebral palsy, developmental delay, fetal alcohol syndrome (or FASD) and other disorders that occur during the developmental period (birth to age 18).
The major differences are in the age of onset, the severity of limitations, and the fact that a person with a developmental disability definition may or may not have a low I.Q. While some people with intellectual disability will also meet the definition of developmental disability, it is estimated that at least half do not meet the requirements for the developmental disability definition.