What is Reaction coordinate, energy profile, transition state?

Along an elementary step the atoms move from the arrangement of the reactants to that of the products; a variable that measures this progress (a bond length that grows, another that shrinks) is the reaction coordinate. The graph of the potential energy of the system along it is the energy profile. Its maximum is the transition state, an arrangement of the atoms with bonds half-made and half-broken, which lasts no longer than a molecular vibration and cannot be isolated. The height of this maximum above the reactants is the energy barrier of the step.