Radial velocity is the velocity of an object in the direction of the line of sight. The light of an object with a substantial radial velocity will be subject to Doppler effect, so the wavelength of the light increases for receding objects (redshift) and decreases for approaching objects (blueshift).
The radial velocity of a star or other luminous but distant object can be measured accurately by taking a high-resolution spectrum and comparing the measured wavelengths of known spectral lines to wavelengths from laboratry measurements.
In many binary stars, the orbital motion usually causes radial velocity variations of several kilometers per second. As the spectra of these stars vary due to the Doppler effect, they are called spectroscopic binaries. Radial velocity studies can be used to estimate the masses of the stars, and some orbital elements, such as eccentricity and semimajor axis. The same method has also been used to detect planets around stars.