Today 'energy' is a household term, as illustrated in this Beetle Bailey cartoon (by Mort Walker, published in the Centre Daily Times, 8/11/96):
The Merriam-Webster Collegiate Dictionary cites its use since 1599, but its precise meaning has been clear only since the 1850s. Historians of science have probably written more essays on the development of the concept of energy than on any other subject. Here is its brief history. (For more details of this exciting story see, for example, the book by H. J. Steffens, "James Prescott Joule and the Concept of Energy", Science History Publications, New York, 1979, or the early pioneering article by George Sarton, "The Discovery of the Law of Conservation of Energy", Isis, Vol. 13, 1929-30, pp. 18-34.)
The concept of energy has evolved from those of the archaic fire, the more modern vis viva ("living force"), which was dominant until the nineteenth century, and force, which persisted well into the 19th century and today has a much narrower scope. It was Aristotle (384-322 BC) who developed the concept of 'fire' as one of four basic 'elements' of nature first described by Empedocles (490-430 BC), the other three being earth, water and air. This view remained unchallenged for the next two thousand years. The largely apathetic and deeply religious 'scientists' of the Dark and Middle Ages did not seem to care to clarify it. The age of Enlightenment had to come eventually, and it did in the late seventeenth century. The inquiry into the how's and the why's of this world was then not only resurrected but it was systematized into what we today know as the scientific method. This allowed the German Leibniz (1646-1716) to champion the idea that the "vis viva" of a body is its mass times the square of its speed (what we now know to be twice the kinetic energy of a body). It also allowed the rapidly growing scientific community to draw the distinction, in the following 150 years or so, between the more abstract concepts of force and energy and the less abstract concepts of heat and work. The invention of the thermometer - as early as in the 1590s by Galileo, who didn't quite know what to do with it, and then by Fahrenheit in the early eighteenth century - helped to clarify the distinction between temperature and heat. Detailed studies of heat by Joseph Black (1728-1799) at the University of Edinburgh inspired James Watt (1736-1819) to develop the first modern steam engine which propelled the Industrial Revolution throughout the nineteenth century and beyond. This crucial technological development in turn inspired the scientific community to clarify the laws governing the conversion of heat to work.
In contrast to the early human realization that mass is conserved in all earthly and heavenly phenomena (except in nuclear reactions, where mass is converted into energy according to the famous Einstein equation), the fact that conservation of energy is an even more basic law of the universe did not become clear until mid-nineteenth century, when the science of thermodynamics was developed. The key players in this story are the Englishmen Thomas Young (1773-1829) and James Prescott Joule (1818-1889), the American-born Benjamin Thompson (1753-1814), and the Germans Robert Mayer (1814-1878) and Hermann Helmholtz (1821-1894). While Young is better known for having demonstrated the wave-like character of light and for his research on the elasticity of materials, he is often credited for being the first, in 1807, to use the word 'energy' in its modern scientific sense. Thompson, better known as Count Rumford (and very popular among historians for his military, political and amorous adventures), clarified the nature of heat by showing in 1804 that it is not a fluid-like substance, as widely believed until then. This realization and the quantitative measurements of energy interconversion by Joule, Mayer and others (in the period 1840-1850), were the key to the establishment of a clear relationship between heat and work, as two qualitatively different but quantitatively equivalent forms of energy. Finally, in 1847 the inspired young Helmholtz generalized this principle of conservation of energy into a universal law of nature, which came to be known as the First Law of Thermodynamics.
There is some irony in these historical developments: just before major social upheavals were to spread throughout most of Europe in the revolutions of 1848, the collective efforts of the European scientific community brought about one of the major intellectual syntheses of all time.
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