A streamline is a
field line in the
velocity field of a
fluid at some instant, ie. a
curve which on all
points along it is in the same direction as the velocity of the fluid at that point and time.
In a steady flow, ie where the velocity field does not change with time, the streamlines show the paths of the fluid elements (it is worth noting the difference that a streamline is a curve in
space while a particle moves in
space-time, and that its path is merely a
projection onto space). In an unsteady flow this is not the case, and the streamlines will also change with time. In any case streamlines give a good view of the instantaneous properties of the fluid motion, in the same way that
magnetic field lines help visualise
magnetic fields etc.