State functions seem to always be described as relating state variables in equilibrium, wikipedia about state functions:
In thermodynamics, a state function, [...] is a function defined for a system relating several state variables [...] that depends only on the current equilibrium thermodynamic state of the system (e.g. gas, liquid, solid, crystal, or emulsion), not the path which the system took to reach its present state.
The German wikipedia about state variables states that if all state variables are constant in time, the system is in thermodynamic equilibrium. Leading me to believe, they may be at least defined outside of equilibrium.
Am I understanding it correctly if I say, that they are only well defined in thermodynamic equilibrium, at least partial equilibrium, meaning thermal, mechanical or chemical respectively, allowing at least the corresponding state variables to be well defined?
Would this be sensible definitions, based on this?
State variable: A Measurable macroscopic quantity that is only well defined, in the sense of describing the state of a system, if it is constant in time. And that is not subject to hysteresis.
State function: A function between state variables, that are well defined and thus in equilibrium.
Where equilibrium just means the state variables in consideration are constant in time and no macroscopic fluxes are occurring. I hesitate to use the thermodynamic equilibrium in the state functions definition, because the zeroth law of thermodynamics only requires thermal equilibrium to measure temperature, so full thermodynamic equilibrium can not always be required.
Clarification: Maybe the question really is: We can measure variables like pressure at any point in time, but does it become a state variables only, when they it is constant in time, so we can be sure, that they represent the average in an equilibrium sense, and not just a short snapshot in time?