Define ‘activation energy’ of a reaction

Activation energy is nothing but the energy required for a chemical reaction to occur.

So the minimum amount of energy required to activate the atoms or molecules of the reactants to a certain stage in which they can take part in the chemical reaction is known as activation energy.

For a reaction to occur, the existing bonds (of the reactants) must break and form new ones (for product). A reaction will only proceed if and only if the products are more stable than the reactants. To get the bonds (of the reactants) into a state that allows them to break, the molecule must be contorted (deformed or bent) into an unstable intermediate state called the transition state.

As the transition state is an intermediate stage and unstable, the reactant molecules do not stay there for long, but quickly proceed to the next step of the chemical reaction.