The fluid flow with zero material derivative of its density:
(1) | \frac{D \rho}{ Dt} = \frac{\partial \rho}{\partial t} + \rho \cdot \nabla {\bf u} = 0 |
With account of Continuity equation:
(2) | \frac{\partial \rho}{\partial t} + \nabla (\rho \, {\bf u}) = 0 |
the Incompressible flow criteria simplifies to:
(3) | \nabla {\bf u} = 0 |
It does not necessarily means that the fluid itself is incompressible.
In many practical applications condition (3) is met for compressible fluids and the fluid flow behaves as Incompressible flow.
See also
Physics / Mechanics / Continuum mechanics / Fluid Mechanics / Fluid Dynamics / Fluid flow