From AMS Glossary
(Or path.) A curve in space tracing the points successively occupied by a particle in motion.
At any given instant the velocity vector of the particle is tangent to the trajectory. In steady- state flow, the trajectories and streamlines of the fluid parcels are identical. Otherwise, the curvature of the trajectory KT is related to the curvature of the streamline KS by
where V is the parcel speed and ∂ψ/∂t is the local change of the wind direction. The curvatures and wind change are positive for the cyclonic sense of flow.