Hammond Postulate Definition:

It is a hypothesis that describes a geometric structure of the transition state of an organic chemical reaction.

Hammond Postulate Explained:

In an exothermic process, the transition state is closer in energy to the reactants than to the products, and therefore the structure of the transition state more closely resembles the reactants. In contrast, the transition state in an endothermic process is closer in energy to the products, and therefore the transition state more closely resembles the products.