In thermodynamics, the state of a system is defined by the ensembly of physical quantities: temperature, pressure, compostion, etc., which characterize the system, but neither by its surroundings nor by its history. [1] The term was first used in a descriptive sense in 1851 by English physicist James Joule who referred to the internal energy of a body as the: [2]


The more detailed description of the state of a given body was described by American mathematical physicist Willard Gibbs in the 1870s. [3] In origins, the term stems from the 17th century work on developing the ideal gas laws and the equations of state that resulted.

References
1. Perrot, Pierre. (1998). A to Z of Thermodynamics, Oxford: Oxford University Press.
2. Joule, James. (1851). Trans. Royal Soc. of Edinburgh, Vol. xx., pg. 475. (as referenced on pg. 31 of Clausius’ 1879 Mechanical Theory of Heat).
3. (a) Gibbs, J. Willard. (1873). "Graphical Methods in the Thermodynamics of Fluids", Transactions of the Connecticut Academy, I. pp. 309-342, April-May.
(b) Gibbs, J. Willard. (1873). "A Method of Geometrical Representation of the Thermodynamic Properties of Substances by Means of Surfaces", Transactions of the Connecticut Academy, II. pp.382-404, Dec.
(c) Gibbs, Willard. (1876). "On the Equilibrium of Heterogeneous Substances", Transactions of the Connecticut Academy, III. pp. 108-248, Oct., 1875-May, 1876, and pp. 343-524, may, 1877-July, 1878.