A star's velocity components. Image credit: CSIRO Australia
The radial velocity is the speed at which an object is moving away from or toward an observer along the line of sight. By convention, it is positive if receding, negative if approaching.
In the case of luminous astronomical objects, radial velocity can be calculated from the displacement of spectral lines from their normal position: a object moving toward us has a blueshifted spectrum, while an object moving away has a redshifted spectrum. The larger the blueshift or redshift, the larger the radial velocity. The present radial-velocity champion for a star in the Milky Way Galaxy is Giclas 233-27 in Lacerta, which is approaching us at 583 kilometers per second. External galaxies and, especially, quasars, may show very large recessional radial velocities due to the overall expansion of the universe. Compare with transverse velocity.