STREAMLINES are a family of curves that are instantaneously tangent to the velocity vector of the flow. These show the direction in which a MASSLESS fluid element will travel at any point in timeStreaklines are the loci of points of all the fluid particles that have passed continuously through a particular spatial point in the past. Dye steadily injected into the fluid at a fixed point extends along a streak line.Pathlines are the TRAJECTORIES that individual fluid particles follow. These can be thought of as "RECORDING" the path of a fluid element in the flow over a certain period. The direction the path takes will be determined by the streamlines of the fluid at each moment in time.For steady flow, path lines, streamlines and streaklines coincide.Stream tube: A bundle of neighboring streamlines may be imagined to form a passage through which the fluid flows. This passage is known as a stream-tube.Properties of stream tube:It is bounded by all sides by the streamlines.Fluid velocity does not exist ACROSS a streamline