A star's total velocity with respect to the local standard of rest. This is the combination of the star's U, V, and W velocities: space velocity = sqrt (U2 + V2 + W2). For example, the Sun (U = -9, V = +12, W = +7) has a space velocity of 17 kilometers per second.
The space velocity of a star is its velocity relative to the Sun or the Local standard of rest. Its components in the Galactic coordinate system are usually designated U, V, and W, given in km/s, with U positive in the direction of the Galactic center, V positive in the direction of Galactic rotation, and W positive in the direction of the North Galactic Pole. These velocity components can be calculated from the observed radial velocity, proper motion and parallax Johnson & Soderblom 1987, Astronomical Journal 93, 864.