∞ generated and posted on 2016.08.20 ∞
Capacity to do work.
If that definition sounds rather cryptic, well there's a reason for that: It is! When broken down into its components, it (perhaps) will make more sense.
In other words, if something is sufficiently difficult to accomplish that it doesn't just happen on its own, then it requires energy to make that thing happen, and the process of making that thing happen we call work.
Energy thus is the actual or potential ability to perform productive or operative activity. Energy, in yet other words, is what makes things happen. No energy, then nothing happens. See also electrons store energy.
Typically one differentiates energy into different types, the most fundamental of which are potential energy versus kinetic energy. See also light energy and chemical energy.
Figure legend: Water prior to a waterfall is at a static equilibrium with regard to gravity, stuck as it is on a shelf of rock. At the edge of the waterfall the potential energy associated with the water's distance from the center of the Earth is converted to kinetic energy, as the water falls towards that center. If a turbine were present, the water's kinetic energy could be converted to electrical energy with this electricity then used to perform work, such as to power the computer upon which I am writing.
Organisms take in energy, change that energy into different forms, and then tap that energy to maintain themselves, grow, repair themselves, make babies, etc. See bioenergetics, which is the formal study of such energy transduction.
The following video "discusses" potential energy versus kinetic energy:
The following video also considers potential energy versus kinetic energy: