Svante August Arrhenius was a chemist who worked out that reactions needed energy to occur. He created a formula for the concept and coined the term ‘activation energy’ in 1889. It’s defined like this:
Activation energy is the energy which must be available to a chemical system with potential reactants to result in a chemical reaction.
A great simple example is that of lighting a match. A match touching the box doesn’t result in spontaneous combustion. You create the activation energy (heat and friction) required for the combustion when you strike the match against the box with enough force.
Activation energy is really interesting when applied metaphorically to other parts of day to day life. One example is looking for ways to reduce the activation energy for anything that you want to do more of, or increase the activation energy for things you want to do less of. (This is why I now put the chocolates in the difficult-to-reach top cupboard!) 🍫