Intellectual disability involves problems with general mental abilities that affect functioning in two areas:

intellectual functioning (such as learning, problem-solving, judgment)

adaptive functioning (activities of daily life such as communication and independent living)

Intellectual disability affects about one percent of the population, and of those about 85 percent have a mild intellectual disability. Males are more likely than females to be diagnosed with intellectual disability.

Sources: 

Atkins v. Virginia | Oyez

Roper v. Simmons | Oyez

Hall v. Florida | Oyez

Atkins v. Virginia - Wikipedia

Hall v. Florida - Wikipedia

Roper v. Simmons - Wikipedia