A streakline consists of all fluid particles
that have previously passed through a specified point. This is essentially
the same as injecting dye or smoke continuosly at a given location
and observing how the dye or smoke moves along with the fluid
motion. This is a technique that is often used in experiments to visualize the flow field.
On the other hand, the actual path that a single fluid particle takes
is referred to as the pathline, i.e., it is the trajectory of a
particular fluid particle. This is referred to as the Lagrangian viewpoint of the flow field. Experimentally,
it can be achieved by tagging a fluid particle and tracing its motion throughout the flow field.
In general, a streamline, streakline and pathline are not the same; however, they coincide when the flow is steady.