Final answer:
A child able to think abstractly, reason deductively, and define abstract concepts is in Piaget's formal operational stage, which encompasses advanced reasoning and abstract thought from around age 11 into adulthood.
Step-by-step explanation:
The child you are describing, who can think abstractly, reason deductively, and define abstract concepts, would fit into Piaget's formal operational stage of cognitive development. This stage, which typically begins at around age 11 and continues into adulthood, is characterized by the ability for more complex thought processes, including abstract reasoning and hypothetical thinking. Unlike children in the concrete operational stage who can only think logically about concrete events, those in the formal operational stage can also deal with abstract ideas and hypothetical situations. This stage encompasses the development of advanced reasoning, problem-solving skills, and the capacity to think about abstract and theoretical concepts.