Acceleration is defined as the rate of change of velocity with time. It is a vector quantity. Its unit is m/s2. Constant speed does not guarantee that acceleration is zero. For example a body moving with constant speed in a circle changes its velocity every instant and hence its acceleration is not equal to zero.
Retardation (or deceleration) of a body whose velocity is decreasing with time is defined as the negative of acceleration. In other words, it is equal to the magnitude of acceleration of a body whose velocity is decreasing with time. For a decelerating body, direction of velocity and acceleration is always opposite in direction. Example: When a body is thrown vertically upwards, during the upward motion, its velocity decreases and at the highest point it becomes zero. This is a case of retardation.
Unit of Acceleration
Acceleration is defined as the rate of change of velocity or velocity per unit time.Its unit is m/s2, can also be expressed as N/kg. Centripetal acceleration:Centripetal acceleration is the rate of change of tangential velocity.