The rate at which something can change its velocity is the ACCELERATION of the object.

If the acceleration is positive the object is speeding up, if it is negative it is slowing down. A negative acceleration is a DECELERATION or a RETARDATION.

Acceleration = change in velocity/Time taken

Acceleration = (Final velocity - Initial velocity)/Time taken

Acceleration is an increase in velocity per second and so the units for acceleration are (metres per second) per second written m/s^{2}