Radial velocity (RV) is the component of velocity of a body on a line between the body and the observer, i.e., the movement toward or away from the observer. (The line between observer and body is a radius of the sphere surrounding the observer at the distance of the body.) A star's radial velocity can be determined by measuring the Doppler shift of known spectral lines (spectral radial velocity), which can be used to identify the presence of extra-solar planets and/or binary stars. Observatories with instruments designed to determine radial velocities are sometimes referred to as RV observatories (radial velocity observatories).
Measuring changes in radial velocity over time is helpful for detecting planets with small (a day or few days) or medium (1-2 AU) orbits (the radial velocity method or RV method of exoplanet detection), but transits reveal more of the smaller orbits. Using the RV method requires HRS, which, in turn, requires a degree of brightness (apparent magnitude of 12 or brighter) and currently measures down to about one meter/second precision. 1/10 meter/second would be required to detect Earth.
Radial velocity represents one component of a body's velocity relative to the observer, covering one of the three spatial dimensions. A two-dimensional velocity covering the other two dimensions (at right angles to the line of sight) is termed transverse velocity or tangential velocity. For nearby stars, peculiar velocity offers a clue to the transverse velocity, which can be pinned down if the distance is known, e.g., using parallax. Transverse velocity does have a relativistic effect on Doppler shift (stemming from time dilation), which can significantly affect RV measurements if the magnitude of the transverse velocity is far greater than the radial velocity.
The term velocimetry is sometimes used for measurement of velocity, and radial velocimetry for the RV measurement.