The generic meaning of transitions—as passages from one condition or action to another—is quite straightforward and hence readily understood, but adding the energy qualifier complicates the comprehension. Energy, a concept that in itself is notoriously hard to define in an easy intuitive manner, encompasses a veritable universe of states and processes, and that is why the term energy transitions deserves some annotation. The focus should be always on a process, not just on its initial and concluding stages, but the most revealing analysis must deal with several key variables and use different measures to trace their change.

There is no formal or generally accepted hierarchy of meanings, but the term energy transition is used most often to describe the change in the composition (structure) of primary energy supply, the gradual shift from a specific pattern of energy provision to a new state of an energy system. This change can be traced on scales ranging from local to global, and a universally experienced transition from biomass to fossil fuels is certainly its best example. Many specific inquiries are possible within this grand shift: For example, the focus can be on transitions from wood to charcoal in heating, from coal to oil in households and industries, from oil to natural gas in electricity generation, or from direct combustion of fossil fuels to their increasingly indirect use as thermal electricity. These studies of changing structure of energy supply often focus on the time elapsed between an introduction of a new primary energy source and its rise to claiming a substantial share (arbitrarily defined) of the overall market, or even becoming the single largest contributor or the dominant supplier on a local, national, or global scale. But given an often impressive growth of energy supply over time, close attention should be also given to absolute quantities involved in the transitions as well as to qualitative changes that result in wider availabilities of energies that are more flexible, more efficient, and more convenient to use even as they create substantially lower environmental impacts. Combination of all of these approaches would provide the best understanding of the transition process.