Adult derives from the Latin adultus: “full-grown, mature, firmly established.” To be an adult, the root implies, is to reach the endpoint of a developmental arc—to attain the physical, behavioral, and social norms of maturity—but this fantasy of fulfillment is tenuous. Adult’s first OED definitions are adjectives, unmoored from a stable subject. Adult might make any person full grown; it also makes an attitude sophisticated or a movie erotic. Adult’s noun form—“a person who is fully developed; one who has reached maturity”—is similarly volatile, its boundaries porous enough to let in its supposed opposite, the child. Half the quotations the OED provides in support of this definition reference children, many aligning adult and child experiences. (Herbert Spencer, for example, wonders “what rights are common to children and adults.”) Compounds yoke adults to ideas associated with young people: adult student, adult illiteracy. The word’s desultory travel between subject and descriptor emphasizes the contradictions at its heart, inviting us to explore the fraught relationships between adult and child.