Mass flux

Mass flux is the mass crossing a unit area per unit time. In a one-dimensional flow it is $\rho u$.

The mass loss rate is the positive rate at which a body loses mass. For a steady spherical stellar wind, the continuity equation gives $\dot{M}=4\pi r^2\rho u_r$, constant across spheres enclosing the source. It is positive for outward radial flow; an accretion convention often uses a separate positive inward rate.