In fluid mechanics, a homentropic flow has uniform and constant entropy. It distinguishes itself from an isentropic or particle isentropic flow, where the entropy level of each fluid particle does not change with time, but may vary from particle to particle.

For an ideal gas, the general equation of an isentrope, relating pressure and density is P = Aρ² In a homentropic flow, the constant A is fixed for all particles at all times.