Velocity is the vector measuring the time rate of change of displacement. It is specified by a magnitude (the speed) and a direction. In general, the velocty is a function of time:

where is the three-dimensional vector displacement, also written as a function of time.

The velocity is expressed in units of [distance/time], often m/s or km/h, with the direction of motion specified as well.

In astronomy, the units of km/s are often appropriate for the velocity of the stars relative to one another.

The component of velocity along the line of sight to an object is the radial velocity, whereas the component perpendicular to our line of sight is the transverse velocity.