Generally it is referred to the changes which occur to a person's cognitive structures, abilities, and processes. The most widely known theory of childhood cognitive development was proposed by Jean Piaget in 1969. He proposed the idea that cognitive development consisted of the development of logical competence, and that the development of this competence consists of four major stages (sensori-motor, preoperational, concrete operational, and formal operational). He also argued that a child's cognitive performance depended more on the stage of development he was in than on the specific task being performed.
More recent studies have cast some doubt on Piaget's theory of homogeneous performance within a given stage. Instead, it is now believed that performance varies greatly within each stage and depends more on the acquisition and development of language, perception, decision rules, and real-world knowledge for any individual child.