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:
| the fluid temperature is fully determined at any point in time and any spatial location of a fluid flow | |
| the fluid pressure is fully determined at any point in time and any spatial location of a fluid flow | |
| fluid density is a function of pressure and temperature only, so that it depends on time and spatial location implicitly | |
| fluid viscosity is a function of pressure and temperature only, so that it depends on time and spatial location implicitly | |
| fluid thermal conductivity is a function of pressure and temperature only, so that it depends on time and spatial location implicitly | |
| fluid isobaric specific heat capacity is a function of pressure and temperature only, so that it depends on time and spatial location implicitly | |
| fluid Joule–Thomson coefficient is a function of pressure and temperature only, so that it depends on time and spatial location implicitly | |
| The Gibbs free energy is at minimum |