In thermodynamics, energy (TR:1984) (LH:21) (TL:2005), from the Greek letter "Ε" (epsilon), meaning "sun", from the Greek word “en-ergon” (εν-εργειας) (Heraclitus, 490BC), meaning “in work” (or at work), refers to 
In 2500BC, in Egyptian hieroglyph of a man with his arms raised, shown below, became, via Proto-Sinaitic and Phoenician cultural transmission, the root etymology of the Greek letter epsilon "E", symbolic of a person ‘in action’, or doing ‘work’ of some sort, i.e. expending ‘energy’ as we would say:
In 1,000BC, during the formation of the Greek alphabet, letter E became, as the letter epsilon (ε or Ε), the second vowel of the seven vowels, thematic to or named after the sun, the second planet in Greek cosmology.
In 800BC, Homer was using the term "erga" in the sense of "work".
In 490BC, Heraclitus, the “flux and fire” philosopher, was referring to en (εν) ergon (εργειας) as the father and king of all things, as follows:
- “En-ergon is the father of everything, king of all things and, out of it, all forms of contrast originate. Since ‘en-ergon’ is common to everything, it is vital for life itself.”
- — Heraclitus (490BC), Fragment #; cited by Gerrit Feekes (1986) in The Hierarchy of Energy Systems (pg. 1) 
Heraclitus, we note, also believed in the existence of the vacuum (or void), contrary to Parmenides, who considered the existence of a void or vacuum to be an anathema. This connects us with the Hooke principle (1675) of thermodynamics.
The isopsephy values, of en-ergon, are “55” (εν), which translates as: “one; with, within”, and “324” (εργειας), which renders as: “pure, sacred, holy”. Thus, en-ergon might crudely render, in secret word meaning, as "one who is with, at, or within work is pure, sacred, or holy", or something to this effect. This might render, accordingly, in what Csikszentmihalyi (1990) refers to as “flow”, meaning a state wherein one is so emersed in their “work”, that a sense of timelessness results.
In 350BC, Aristotle, building on Heraclitus, was using the term "energia", in the sense of "to act".
In 1807, Thomas Young defined energy in the sense of what we now refer to as kinetic energy:
- “Since the height, to which a body will rise perpendicularly, is as the square of its velocity, it will preserve a tendency to rise to a height which is as the square of its velocity, whatever may be the path into which it is directed, provided that it meets which no abrupt angle. The same idea is somewhat more concisely expressed by the term energy, which indicates the tendency of a body to ascend or to penetrate to a certain distance, in opposition to a retarding force.”
- — Thomas Young (1807), “On Confined Motion”
- The term energy may be applied, with great propriety, to the product of mass or weight of a body, into the square of the number expressing its velocity. Thus, if the weight of one ounce moves with a velocity of a foot in a second, we call its energy 1; if a second body of two ounces has a velocity of three feet in a second, its energy will be twice the square of three, or 18.”
- — Thomas Young (1807), “On Collisions”
In 1865, Rudolf Clausius, building on Heraclitus, Young, William Thomson, and many others, defined the "energy" (or internal energy) of a body, symbol U, which subsumed the concept of kinetic energy, thereby defining the energy of bodies or systems, generally, via of the first law of thermodynamics.
The following are related quotes:
- “The term energia was first used by Heraclitus to connote fire as the primary source of action. Heraclitus, in his Physics, considered energon the father of everything and the originator of all life on Gaia.”
- — Paris Arnopoulos (1993), Sociophysics: Cosmos and Chaos in Nature and Culture (pg. 21) 
- Thims, Libb. (2020). Human Chemical Thermodynamics — Chemical Thermodynamics Applied to the Humanities: Meaning, Morality, Purpose; Sociology, Economics, Ecology; History, Philosophy, Government, Anthropology, Politics, Business, Jurisprudence; Religion, Relationships, Warfare, and Love (§2: Alphabet) (pdf) (§36: Joule [§§36.1: | E = Sun (etymology)]). Publisher.
- Plutarch. (c.110), ‘De E apud Delphos’. Publisher.
- (a) Feekes, Gerrit B. (1986). The Hierarchy of Energy Systems: from Atom to Society (pg. 1). Pergamon Press.
(b) Lancaster, Justin. (1989). “The Theory of Radially Evolving Energy” (abs), International Journal of General Systems, 16: 43-73.
- Barry, Kieren. (1999). The Greek Qabalah: Alphabetic Mysticism and Numerology in the Ancient World (pdf) (#55, pg. 219; #324, pg. 229). Weiser.
- Flow – Hmolpedia 2020.
- Arnopoulos, Paris. (1993). Sociophysics: Cosmos and Chaos in Nature and Culture (pg. 21). Nova Publishers, 2005.
- Energy – Hmolpedia 2020.