Definitions and associated descriptions of the condition now commonly known as "intellectual disability" serve many functions. The "Atkins v. Virginia" U.S. Supreme Court decision (2002) has called attention to the importance of clear, objective, and measureable wording of the definition. This article discusses the potential for misunderstanding and misinterpretation of such words as "ability" and "cognitive," the importance of clarifying the role of measurement error and sociocultural factors, and the noncausal relationship between impairment in intelligence and adaptive behavior.