The increase in the velocity of an object per unit of time is known as acceleration. For example, free-falling stone. The decrease in the velocity of an object per unit of time is known as retardation. For example, the velocity of a vehicle decreases when the brake is applied.
Acceleration can be positive, zero, or negative. Negative acceleration is known as retardation.
The SI unit of both acceleration and retardation are m/s2.