Activation refers to the property that a substance or molecule has the ability to undergo some physical and chemical changes.

For example, an activated molecule refers to a molecule that crosses the peak of energy, which is called an activated molecule. In the reaction system, not all reactant molecules can be converted into product molecules. Once the extra energy not lower than the activation energy is obtained, it has the activity of chemical reaction and becomes an activated molecule.

Another example is that activated natural killer cells (activated NK cells) are activated natural killer cells with activation energy.