Streamline
A curve whose tangent at every point coincides with the local fluid velocity. In steady flow, streamlines are also particle paths.
Definition
A streamline is a curve that, at every point along its length, is tangent to the velocity vector of the fluid at that point. In steady flow — where velocities do not change with time — streamlines coincide with the paths of individual fluid parcels. In unsteady flow they are instantaneous snapshots and differ from particle trajectories.
Streamlines cannot cross: at any point in a fluid the velocity has a single direction, so exactly one streamline passes through. Dye injected into a flow, or smoke in a wind tunnel, visualises streamlines directly. Bernoulli's principle applies along a streamline, not between streamlines in general.