To overcome an energy barrier between reactants and products, energy must be provided to get the reaction started. This energy, which is recovered as the reaction proceeds, is called:

Activation energy

There is an energy barrier that separates the energy levels of the reactants and products. Energy must be added to the reactants to overcome the energy barrier, which is recovered when products are formed. The energy barrier is known as Ea, the activation energy. The activation energy is distinct from the DG, or free energy difference between the reactants and products.