Energy is actually rather hard to define---all textbooks seem to admit this.

1. No short definition is adequate. But a common one-sentence definition that is useful is:
   "Energy is the quantified capacity for change/transformation."

   This is reasonable since loosely speaking, how much energy is available sets a limit on how much change/transformation is possible

   Another one-sentence definition that yours truly made up is:
   "Energy is the transformable and conserved universal essence of structure."

   This definition is also useful.

   Energy has many forms and all are transformable into other forms---NOT necessarily easily---and energy is a conserved quantity. The transformation between energy forms is done by forces: e.g., the electromagnetic force and the gravitational force.

   Calling energy the "essence of structure" also makes sense since a useful description of a physical structure can be given in terms of the various amounts of energy it has. Note, "useful descripton", NOT "complete description".

   Two other points need to be made:
   1. Individual forms of the energy can ONLY be calculated from formulae using direct observables (i.e., direct in some sense) as far as yours truly knows---except total energy in a volume can be calculated from total mass in that volume using E=mc² (see Relativity file: e_mc2_4.html).
   2. Energy conservation in a simple sense does NOT always hold. This difficult refinement is also discussed in Relativity file: e_mc2_4.html. But in everyday life and almost all astrophysics, energy conservation does hold.

   As the discussion above suggests, energy is a somewhat abstract thing---but we're used to abstract things like money.

2. The concept of energy was implicit in Newtonian physics as formulated by Isaac Newton (1643--1727), but he did not notice it. Others in his day had inklings of it, but only in the 19th century did energy and energy conservation become established.

   What good is energy?

   Energy methods allow many kinds of analysis that harder by other methods.

   They often give you partial information easily. For example, something can happen if there is enough energy, but not otherise.

3. A pioneer of energy conservation was James Joule (1818-1889) for whom the joule (J) is named. He was by profession a brewer. The familiar watt (W) is a joule per second.

   By the by, there has always been some mystery about the pronunciation of the surname Joule and even Joule's contemporaries were uncertain about the pronunciation of his name. In fact, the mystery was even used in an advertisement for his beer: "However you pronounce it, it tastes great." (quoted from memory). Which mystery led yours truly to versification:

   The unit of energy is the joule
   and this rhymes with drool,
   but it should rhyme with bowel
   to be correct for James Joule.