Transition state

 A Transition state is the stage in a reaction where molecules involved exist in a transitory molecular structure where they are no longer substrate but have not yet formed products.
The Transition state is the most unstable stage in a reaction pathway this is because it has the highest free energy of all stages in the reaction. For this reason the difference between the free energy of the substrate and the Transition state is called the Gibb’s free energy of activation or the Activation energy.
When calculating the total free energy of a reaction the Activation energy is not included in the final equation as the energy required to reach the Transition state is released as soon as the product forms⁽¹⁾.

1.Berg JM, Tymoczko JL, Gatto Jr GJ, Stryder L. Biochemistry. 8th Ed. W H Freeman. 2015