Streamline
Definition
An imaginary line drawn through a flowing fluid showing the path the fluid follows — like the streaks you see when smoke drifts past an object in a wind tunnel.
Technical definition
A curve whose tangent at every point is parallel to the local velocity vector of the flow at a given instant. In steady flow, streamlines coincide with the actual paths of fluid particles, and no fluid crosses a streamline.
Example
Wind-tunnel photos of cars show streamlines hugging the body shape. Where streamlines bunch together, the fluid speeds up (and by Bernoulli's principle, pressure drops) — designers use this to spot drag-producing regions.
Streamlines are snapshots of the flow direction at one instant, not the trails of individual particles over time — those are pathlines, and they only match streamlines in steady flow. In unsteady, changing flow the two differ.