In chemistry, the transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest potential energy along this reaction coordinate.  It is often marked with the double dagger (‡) symbol.
As an example, the transition state shown below occurs during the S2 reaction of bromoethane with a hydroxide anion:
The activated complex of a reaction can refer to either the transition state or to other states along the reaction coordinate between reactants and products, especially those close to the transition state.
According to the transition state theory, once the reactants have passed through the transition state configuration, they always continue to form products.