Streamline

A streamline is a curve tangent everywhere to the instantaneous velocity field. In steady two-dimensional incompressible flow, streamlines are level sets of the stream function.

A stream tube is a narrow tube whose side boundary consists of streamlines. In steady flow no fluid crosses its sides, so continuity makes its mass flux $A\rho u$ constant. If flow follows a magnetic field with constant magnetohydrodynamic mass loading, its normal area obeys $A|\mathbf{B}|=\mathrm{constant}$.