Boussinesq approximation কী?

The Boussinesq approximation writes the density as $\rho = \rho_0 + \rho'$, with $\rho_0$ a constant and $|\rho'| \ll \rho_0$, and keeps $\rho'$ only where it is multiplied by $g$. The continuity equation becomes $\nabla\cdot\vec u = 0$, and the momentum equation

where $p'$ is the departure of the pressure from the hydrostatic pressure of the density $\rho_0$, $\vec k$ the upward unit vector and $b$ the buoyancy.