In chemistry, the energy required to change from one chemical state to another. You might also encounter a similar phenomenon in physical change called "latent heat" - the energy needed to complete a phase change from one physical state to another.
How it applies
It may seem at first glance that everything we experience is linear. For example, when we add heat to water, we see the temperature increase.
Activation energy, however, is a non-linear curve required to create some type of change. This energy isn't lost or wasted, but is required for the change to take place.
This applies to habit forming, as an example; once you take the first step towards exercising, you are more likely to continue. This is also where the phrase "the first step is always the hardest" comes from.
This could apply to career planning as well; getting your first job in an industry is likely harder than getting your second job.
See also: [[Conservation-of-resources]]