The average acceleration 
of a particle as it moves from one position to another is defined as the change in the instantaneous velocity vector 
divided by the time 
during which that change occurred. The SI unit for acceleration is the metre per second square (m s–2).
The instantaneous acceleration 
is defined as the limiting value of 
as approaches zero:
Deceleration, or retardation, is the negative of acceleration. Acceleration and retardation are both vector quantities.
