The word energy may be the most abused, orphaned and overused term in the entire scientific lexicon. 

 We have the canonical energy of traditional mechanics and chemistry: kinetic energy, potential energy, binding energy, etc..  And we also have some really difficult to grasp notions such as dark-energy, zero-point energy, vacuum-energy, etc..  And finally, we have vernacular to esoteric uses of the word:  energy for exercise or studying, we well as spiritual type energies.  There seems to be no bounds to the applications of this concept.  Interestingly, all of these variations on energy are referent to the concept itself:  that is, they are all extensive phrases that require us to understand the base-word, energy, in the first place.  

So what does this critical word actually mean for science?  Is it possible to define the term such that it can be applied robustly in each situation without any crossover contradictions, as is necessary for objective communication?  I believe so.  However, to accomplish that feat we have to dig into the history of the development of the notion.  The term was first introduced by Thomas Young in 1807, as a replacement for Leibnitz’ “vis-viva” or “life-force.”  Energy came from the ancient Greek word for activity.  

Leibnitz had noticed that it appeared general that the quantity of a body’s motion multiplied by its velocity squared remained constant.  This relation differs from the modern conception of kinetic energy only by a factor of one-half and laid the groundwork for the present-day conception of energy conservation.  Engineers such as James Joule found the notion of energy quite practical and noted that this conserved quantity called energy corresponds well with the ability of the system to perform mechanical work.
 
Taken together, 

energy simply symbolizes the quantity of material in some specific degree of motion.  

This is essentially the same idea as momentum:  some particular thing is moving with some particular speed.  Whenever we see the word energy used in science, we can rest assured we are talking about something moving somehow.  This is an important distinction for physics, in particular, because so much of the modern knowledgebase is relational rather than rational.  In other words, we have astonishingly accurate descriptions of dynamics concerning phenomena but a rather poor grasp of physical actors performing the motions.  In a recent article, “What Warps When Bodies Bend Spacetime?”, I expanded on this idea with respect the phenomenon of gravity.   While the behaviors of bodies and devices like clocks and in the presence of large bodies have been accurately logged, no physical cause has been advanced.  Similarly, the word energy will often detail exact dynamic relations but fail to indicate any of the physical actors involved.

For example, let’s consider the notion of light-energy.  While it is apparent that light is a manner by which materials like stars inflict effort upon their surrounding and that it is also a manner of performing work as in a flashlight, it is unclear what physical actors are being moved during this phenomenon.  We often encounter the description of a photon propagating in a wavelike fashion and might conclude that this photon is the physical actor, but in fact this cannot be the case as the photon does not actually appear to convey material.  Note that the photon does not have mass. Instead, the photon is more reasonably described as an event that occurs as a handshake between two atoms, where one pushes upon another.  In that sense,

light-energy can only be deduced to mean that one atom acted upon the other in the dynamic fashion characterized by the concept of the photon.  To understand how that energy is exchanged, we will need to eventually hypothesize and identify the physical mediators.

Demystifying the photon is a heavy job for another day, but hopefully we have seen how the term energy can obfuscate underdeveloped aspects of our contemporary understanding.  For instance, we have few candidates for what smaller-than-atomic structures convey light-energy, dark-energy, or vacuum-energy.  In order for science to progress and discoveries to unfold, it is first necessary for the next generation of investigators to identify such topics in need of attention.  Fortunately for all potential scientists, rationalization of energetics in terms of physical actors is a corner of physics ripe for immediate attention!