activation energy (Ea)

The excess free energy over the ground state that must be acquired by an atomic or molecular system in order that a particular process can occur.

NOTE The activation energy is used in the Arrhenius equation for the thermal acceleration of physical reactions. The term "activation energy" is not applicable when describing thermal acceleration of time-to-failure distributions.

References: 

JEP122C, 3/06
JESD85, 7/01
JESD91A, 8/01