velocity

The rate of change of position. A vector — both a magnitude (speed) and a direction. In one dimension, `v = dx/dt`; in two, the velocity vector has components `(dx/dt, dy/dt)`. Velocity that does not change in time is called _uniform_; the position then increases linearly. Velocity that changes corresponds to nonzero acceleration. The distinction between speed (a number) and velocity (a vector) is the difference between "fast" and "fast in which direction."