Meaning of "energy of activation"

the minimum quantity of energy that the reacting species must possess in order to undergo a specified reaction.

ac·ti·va·tion energy

ac·ti·va·tion energy (ăk′tə-vā′shən)
The least amount of energy needed for a chemical reaction to occur. For example, striking a match on the side of a matchbox provides the activation energy, in the form of heat, necessary for the chemicals in the match to catch fire.