The Concrete Operational Stage (7 - 11 years) (Piaget) refers to the 3rd of the four (4) stages Piaget uses to define cognitive development.
This stage typically occurs between the ages of 7 and 12. During this stage, the child begins to reason logically, and organize thoughts coherently, however, they can only think about acutal physical objects; they cannot handle abstract reasoning; its also characterized by a loss of egocentric thinking. During this stage, the child has the ability to master most types of conservation experiments, and begins to understand reversibility. The concrete operational stage is also characterized by the child's ability to coordinate two dimensions of an object simultaneously, arrange structures in sequence, and transpose differences between items in a series.