Scottish engineer William Rankine's 1859 explanation of how curve AC is an adiabatic curve, representing an expansion of a substance made without receiving or emitting heat. [3]

In thermodynamics, adiabatic refers to a system undergoing a process in which only work is exchanged with the surroundings. [1] The basic model of the term is that in which a substance expands, pushing on the piston, in such a way that no heat transfers across its boundary, whereby all of the energy of the expanding body is transferred to the piston as work (via pressure volume work).

Etymology
The term “adiabatic”, consisting of the double Greek prefixes: a- (not) and dia- (through or across), thus referring to boundaries in which no heat can pass, was introduced in 1859 by Scottish engineer William Rankine, in his A Manual of the Steam Engine and Other Prime Movers, referring to a special class of pressure-curves corresponding to the case of expansion within an envelope impermeable to heat. [2]

References
1. Perrot, Pierre. (1998). A to Z of Thermodynamics (adiabatic, pgs. 6-7). Oxford: Oxford University Press.
2. (a) Clausius, Rudolf. (1879). The Mechanical Theory of Heat (pg. 68). London: Macmillan & Co.
(b) Truesdell, Clifford. (1980). The Tragicomical History of Thermodynamics, 1822-1854 (pg. 33). Springer-Verlag.
3. Rankine, William. (1859). Manual of the Steam Engine and Other Prime Movers (adiabatic, pgs. 302, etc.) London: Charles Griffin and Co.

External links
● Adiabatic – Wikipedia.