The path followed by fluid particles is called line of flow.
If every particle passing through a given point of the path follows the same line of flow as that of preceding particles, the flow is said to be streamlined. A streamline can be represented as the curve or path whose tangent at any point gives the direction of the liquid velocity at that point.
In steady flow, the streamlines coincide with the line of flow.
Streamlines do not intersect each other because two tangents can then be drawn at the point of intersection giving two directions of velocities, which is not possible.
When the velocity of flow is less than the critical velocity of a given liquid flowing through a tube, the motion is streamlined.
If the velocity of flow exceeds the critical velocity, the mixing of streamlines takes place and the flow path becomes zig-zag. Such a motion is said to be turbulent.