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.
See also
Physics / Mechanics / Continuum mechanics / Fluid Mechanics / Fluid Dynamics / Fluid flow