In a chemical reaction, the stage during which atoms have the highest energy is the transition state. According to transition state theory, the transition state is a hypothetical intermediate phase that exists between the reactants and products during the course of a reaction. In a diagram showing reactant and product energies versus the reaction's course, the top part of the curve that forms is called the transition state.
In this diagram, the difference in product and reactant energies is called the activation energy. During the course of a reaction, the transition phase is a state in which a species that is neither solely reactants nor products forms. This species is called the activated complex, which also is referred to as a higher-energy species.