@wikipedia


A state of a thermodynamic system near to thermodynamic equilibrium in a sense that at any moment of time  and any spatial location  there is a small area of space where fluid can be considered as being at thermodynamic equilibrium during some period of time.


Particularly in fluid dynamics it means that:


T=T(t, {\bf r})



the fluid temperature is fully determined at any point in time and any spatial location of a fluid flow


p=p(t, {\bf r})



the fluid pressure is fully determined at any point in time and any spatial location of a fluid flow


\rho = \rho(p, T)=\rho(p(t, {\bf r}), T(t, {\bf r}))



fluid density is a function of pressure and temperature only, so that it depends on time and spatial location implicitly


\mu = \mu(p, T)=\mu(p(t, {\bf r}), T(t, {\bf r}))



fluid viscosity is a function of pressure and temperature only, so that it depends on time and spatial location implicitly


\lambda = \lambda(p, T)=\lambda(p(t, {\bf r}), T(t, {\bf r}))



fluid thermal conductivity is a function of pressure and temperature only, so that it depends on time and spatial location implicitly


c_p = c_p(p, T)=c_p(p(t, {\bf r}), T(t, {\bf r}))



fluid isobaric specific heat capacity is a function of pressure and temperature only, so that it depends on time and spatial location implicitly


\alpha = \alpha(p, T)=\alpha(p(t, {\bf r}), T(t, {\bf r}))



fluid Joule–Thomson coefficient is a function of pressure and temperature only, so that it depends on time and spatial location implicitly


G \rightarrow \rm min \ \Leftrightarrow \mathrm{d}G = 0



The Gibbs free energy is at minimum