What is a streamline in a flow?

A Streamline a line drawn tangential to the velocity vector.  There cannot be a flow normal to a streamline.

The equation for streamlines is du/u = dv/v = dw/w

A collection of stream lines is called a streamtube.

What is a Pathline in a flow

A pathline is a path traced by an individual particle in a flow.

A streakline consists of fluid particles which have passed a specific point or line.

A Timeline describes adjacent particles at a given moment in time.