The average velocity of a mass point during a given time interval is defined as the ratio of the masspoint's displacement and the time elapsed during the time interval.

Definition of average velocity in symbols:

In this definition, Δ denotes the change in the position vector, i.e., the displacement, of the mass point during the given time interval of duration Δt.

Average velocity is a vector quantity because it is the ratio of a vector and a scalar. The direction of the average velocity vector is that of the displacement vector Δ.

The SI-unit of average velocity is m/s.

Note. The magnitude of the average velocity is denoted |_av|, not v_av. The latter symbol denotes the average speed, which is defined as the ratio s/Δt of distance s traveled and time elapsed and is in general not equal to the magnitude of the average velocity which is equal to |Δ|/Δt. The reason is that distance traveled is measured along the path actually traveled by the masspoint while the magnitude of the displacement is equal to the straight-line distance between the positions of the masspoint at the beginning and end of the given time interval. See the illustration below.

The initial and final positions of a red ball are indicated by the blue position vectors _i and _f, respectively. The magnitude of the ball's displacement |Δ| is equal to the magnitude of the pale green displacement vector while the distance s traveled by the ball is measured along the curved blue path taken by the ball in going from the initial to the final position.