Along a streamline flow of fluid

In fluid mechanics, a streamline is defined as a curve whose tangent at any point gives the direction of the fluid velocity at that point. In a streamline or steady flow, the velocity of the fluid at any fixed point in space remains constant over time. This means that every fluid particle arriving at a specific position will have the same velocity (both magnitude and direction) as the particle that preceded it at that same position. While the velocity of a single particle may change as it moves from one point to another along the streamline—due to changes in the flow field's geometry or pressure gradients—the velocity of all particles crossing a particular fixed position remains invariant. Therefore, the defining characteristic is the constancy of velocity for all particles passing through a given point.