The his thermodynamics book, Zemansky refers to an example in which a gas in a piston is expanding for which the piston is accelerating.
It is obvious that no equation of state exists for the states traversed by a system that is not in mechanical and thermal equilibrium, since such states cannot be described in terms of thermodynamic coordinates referring to the system as a whole. For example, if a gas cylinder were to expand and to impart to a piston an accelerated motion, the gas might have, at any moment, a definite volume and temperature, but the corresponding pressure, calculated from an equation of state, would not apply to the system as a whole. The pressure would not be a thermodynamic coordinate, because it would depend not only on the velocity and acceleration of the piston but would also vary from point to point.
Extracted from "Heat and Thermodynamics" by Zemansky and Dittman, page 32.
He goes on to state that the pressure is not a thermodynamic coordinate in this case.
How is it possible to state that pressure is not a thermodynamic variable in case of an accelerating expansion of a gas ?