The activation energy is a term coined by the Swedish scientist, Svante Arrhenius in 1889. It means the amount of energy expressed in joules that is required to convert the molecules in one mole of a reactant from a ground state to the transition state. It can also mean the energy that an atomic system must have before an emission or chemical reaction can occur. The activation energy of a reaction may be denoted by Ea.
In relation to biology (such as biochemistry), the activation energy (or energy of activation) pertains to the energy needed to initiate a reaction. For instance, the activation energy required to breakdown glucose into pyruvic acid in respiration is two ATP.
Accordingly, enzymes speed up chemical reactions by reducing the activation energy of a reaction. Moreover, this reduction by an enzyme allows biological reactions to proceed rapidly at relatively low temperatures tolerable by living organisms.1
1P. J. Russell. (2008). "Biology: The Dynamic Science, Volume 1." Canada (Belmont): Cengage Learning. ISBN 9781111795559.