Deceleration refers to a situation where the rate of change of the velocity is negative. If the velocity of an object at time `t_0` is `V_0` and the velocity at time `t_0 + h` is `v_1` , and it is decelerating, the value of `v_1` is less than that of `v_0` .
If an object is decelerating `(v_1 - v_0)/(t_0 + h - t_0) < 0`
The term can be used without ambiguity.
Deceleration means that an object's velocity decreases at an increasing rate, or (perhaps more clearly) when an object's acceleration is less than 0. You might think of it as negative acceleration.
An object with an acceleration of -3cm/sec^2 would be decelerating.
An object with velocities (measured at every second): 100 mph, 90 mph, 70 mph, 40 mph, 0 mph is also decelerating.