Flow Lines: In the Eulerian description, three types of curves are commonly used to describe fluid motion:

• Streamline: A streamline is a curve that is instantaneously tangent to the fluid velocity throughout the flow field.
• Pathline: A path line is the trajectory of a fluid particle of fixed identity.
• Streakline: A streak line is the curve obtained by connecting all the fluid particles that will pass, or have passed through, a fixed point in space.

These are defined and described here assuming that the fluid velocity vector, \(\mathbf{u}\), is known at every point of space and instant of time throughout the region of interest. These curves coincide when the flow is steady and are often valuable for understanding fluid motion.