Streamlines, streaklines and pathlines are field lines in a fluid flow. They differ only when the flow changes with time, that is, when the flow is not steady. Streamlines are a family of curves that are instantaneously tangent to the velocity vector of the flow.

A streamline is a curve that is everywhere tangent to the instantaneous local velocity vector. The streamlines can indicate the direction of flow of the fluid instantaneously in a flow field.

A special shape of the bodies which reduces the fluid friction is called as the streamlined shape. Example: Aquatic animals like fish, crocodile and sharks and the vehicles like cars, flights, ships ect. Streamlined shapes help reduce the amount of air drag when an object is moving.

Bus is not streamlined shape. It is a cuboid shape.

In steady flow, since there is no change in the flow pattern, a streak line is same as a streamline and the path line of a particle.

Streamlines provide instantaneous image of the fluid flow, whereas streaklines give information about fluid flow over certain time frame.

: an uninterrupted flow (as of air) past a solid body in which the direction at every point remains unchanged with the passage of time : laminar flow — compare turbulent flow.

Genomic streamlining is a theory in evolutionary biology and microbial ecology that suggests that there is a reproductive benefit to prokaryotes having a smaller genome size with less non-coding DNA and fewer non-essential genes.

A streamline is a line that is tangential to the instantaneous velocity direction (velocity is a vector, and it has a magnitude and a direction). To visualize this in a flow, we could imagine the motion of a small marked element of fluid. There are other ways to make the flow visible.

A streamlined body is a shape that lowers the friction drag between a fluid, like air and water, and an object moving through that fluid. It offers a minimum resistance to air and water by its particular type of body shape .

Streamlines, streaklines, and pathlines. The direction the path takes will be determined by the streamlines of the fluid at each moment in time. Timelines are the lines formed by a set of fluid particles that were marked at a previous instant in time, creating a line or a curve that is displaced in time as the particles move.

Pathlines are flow paths that the fluid particles take while flowing. These flow paths are also called trajectories. Pathlines can be made visible by introducing light particles into the fluid, which follow the fluid almost without inertia. The figure below shows schematically the trajectories in a laminar and a turbulent flow.

The pathline of a fluid element A is simply the path it takes through space as a function of time. An example of a pathline is the trajectory taken by one puff of smoke which is carried by the steady or unsteady wind.

Use the same point as you used for the path and streamline case. You will see that the streak line for the time independent vector field coincides with the path line starting at the chosen point. In summary, for time independent vector field, the path lines coincide with the streak lines, and they coincide with part of the streamlines.