In thermodynamics, equivalence-value, from the German Aequivalenzwerth, is the mathematical value of a transformation, of heat into work or work into heat, defined by the ratio of the quantity of heat Q, interring or leaving a system, and the absolute temperature T of the surface of system, where the heat enters or leaves: 
The term and logic behind the term were developed in 1854 by German physicist Rudolf Clausius, which is a precursory synonym for the 1865 quantity of entropy, being built on the phenomenon of the mechanical equivalent of heat in relation to its effect inside of working body during a cycle. 
1. Clausius, R. (1865). The Mechanical Theory of Heat – with its Applications to the Steam Engine and to Physical Properties of Bodies, (pgs. 121-27) (Forth Memoir). London: John van Voorst, 1 Paternoster Row. MDCCCLXVII.
2. Clausius, R. (1854). “On a Modified form of the Second Fundamental Theorem in the Mechanical Theory of Heat”, Poggendoff’s Annalen, December. Vol. xciii. p. 481; translated in the Journal de Mathematiques, vol. xx. Paris, 1855, and in the Philosophical Magazine, August 1856, S. 4. Vol. xii. p. 81.