A state of a thermodynamic system which is in thermal, mechanical, and chemical equilibrium.

Specifically it is achieved at:

Maximum Entropy	Minimum Helmholtz free energy	Minimum Gibbs Free Energy

for isolated system	for a system with	for a system with

Particularly in Fluid Dynamics it means that:

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

The Gibbs free energy is at minimum

T= \rm const

the fluid temperature is constant at any moment of time and spatial location of a fluid flow

p=\rm const

the fluid pressure is constant at any moment of time and spatial location of a fluid flow

\rho = \rho(p, T)=\rm const

fluid density is a function of pressure and temperature only

\mu = \mu(p, T)=\rm const

fluid viscosity is a function of pressure and temperature only

\lambda = \lambda(p, T)=\rm const

fluid thermal conductivity is a function of pressure and temperature only

c_p = c_p(p, T)=\rm const

fluid isobaric specific heat capacity is a function of pressure and temperature only

\alpha = \alpha(p, T)=\rm const

fluid Joule–Thomson coefficient is a function of pressure and temperature only

See also