In thermodynamics, internal energy or simply “energy” of a body, symbol U, is a state quantity that expresses the sum of the increment of the heat actually existing in a body in the form of vis viva or molecular motion and of the heat consumed in internal work, when the body changes, from a given initial reference point, due to the addition of an amount of heat Q imparted to the body. The expression for internal energy, as defined in 1850 by German physicist Rudolf Clausius, is: [1]

U = H + J

where H is the “quantity of heat” actually existing in a body, i.e. the vis viva of its molecular motions, and J is the ergal or heat consumed in doing internal work (the work which the internal forces must have performed), while the body was passing from any given initial condition, taken as a starting point, to its condition at the moment under consideration. The internal energy depends only on the condition of the body at the moment, and not on the way in which it arrived at that condition. [2]

Etymology
In 1875, Clausius compares his U function to similar variations given by other authors on the subject of heat: Scottish physicist William Thomson (1851) calls it the “mechanical energy of a body in a given state”, German physicist Gustav Kirchhoff calls it “function of activity” (wirkungsfunction), and German physicist Gustav Zeuner calls it “Interior heat” (1860) of the body as well as “internal work” (1866) of the body. [2]

References
1. Clausius, Rudolf. (1850). "On the Motive Power of Heat, and on the Laws Which can be Deduced From it for the Theory of Heat." Poggendorff's Annalen der Physik, LXXIX, 368, 500.
2. Clausius, Rudolf. (1879). The Mechanical Theory of Heat, (pgs. 31-32). London: Macmillan & Co.