•    The curve that gives an instantaneous picture of the location of the fluid particles, which have passed through a given point, is called as streak line.

•    For example, the line formed by smoke particles ejected from nozzle or smoke coming out from chimney. A colour dye when injected into the flowing fluid and a resultant colored filament lines at a given location gives streak lines.

•    The actual path followed by a fluid particle as it moves during a period fo time is called as path line.

•    The path line shows the direction of the velocity of same particles at successive instant of time.

•    A path line can intersect itself.

•    In steady flow, since there are no fluctuations of velocity, path line coincides with streamline.

•    In unsteady flow, the streamline changes their position at every instant and thus the path line may fluctuate between different streamline during an interval of time.

Stream Line:

•    The imaginary line drawn in the fluid in such a way that the tangent to any point gives the direction of motion at that point, is called as stream line.

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